Neurocranium (fig. 2):

The neurocranium is a solidly ossified case with the widest part in the occipital region from where it tapers to the snout.

The ethmoid region consists of four bones. The elongate nasal covers the olfactory organ dorsally. It is sutured to the lateral ethmoid, which forms the caudal and dorsolateral wall of the nasal capsule. The lateral ethmoid has developed an anterior dorsal lamina of membrane bone, which, along with the nasal, forms a rigid cover for the nasal organ (fig. 2). Where the lateral ethmoids meet in the midline, an elongate block of cartilage, a remnant of the trabecula communis, extends back into the orbit (fig. 2B). The ventral corner of the lateral wing of the lateral ethmoid bears an articular facet for the lacrimal, the only bone of the circumorbital series developed in chaudhuriids. The lacrimal forms the lateral wall of the nasal capsule (fig. 2B). Its posteriormost part has an ascending process that contacts the posterior part of the dorsal anterior lamina of the lateral ethmoid. Slightly anterior to this, the lacrimal has an incisure that represents the ventral rim of the posterior nasal opening. The anterior dorsal lamina of the lateral ethmoid forms its dorsal rim. In front of the posterior nasal opening the lacrimal contacts the anterior lamina of the lateral ethmoid and the nasal bone, so that the orifice is completely surrounded by bone. In specimens smaller than the one figured, the respective bones may leave narrow gaps between each other.

Medially the nasals are separated by the unpaired mesethmoid, a thin, vertical, bladelike bone whose anterior end is fused to the vomer (fig. 2B). The mesethmoid is connected ventrally by a short cartilaginous rod to the area where the lateral ethmoids meet in the midline. This cartilage is a remnant of the narrow nasal septum found in other chaudhuriids and in mastacembelids (see Britz, 1996). A conspicuous rodlike rostral cartilage articulates with the anteriormost dorsal part of the mesethmoid (fig. 2A, B). The proximal part of the cartilage, which lies anterior to the tip of the vomer and between the heads of the maxillae, may ossify in larger specimens. The vomer underlies the ethmoid region as a thin splint of bone. Posteriorly the vomer tapers to a needle-thin process located in a groove of the parasphenoid and extends along half the length of that bone (fig. 2C). The anterior end of the vomer, which is indistinguishably fused to the mesethmoid, is enlarged and bears two articular facets for the maxillary bones.

The orbital region, as the area between the otic and the ethmoid part, is dominated in dorsal aspect by the large frontal, which extends from the anterior end of the otic region to the anterior part of the orbit (fig. 2A, B). The paired frontals are sutured to each other along most of their length and taper toward their anterior end where they contact the nasals. There is no trace of a cephalic sensory canal within these bones. The frontal is bordered ventrally over most of its length by the autosphenotic, and it contacts the prootic anteriorly and pterotic posteriorly (fig. 2B). Caudally the frontal reaches the large supraoccipital. The autosphenotic is an elongate bone tapering toward its anterior end. Its posterior part contributes to the anterior articulation for the hyomandibular and in larger specimens it has a ridge from which the levator arcus palatini originates. There is no pterosphenoid or basisphenoid developed.

The prootic is the dominant bone in the lateral wall of the cranium (fig. 2B, C). It is sutured dorsally to the autosphenotic over most of its length, and only its most anterior tip contacts the frontal. Ventrally at the posterior two-thirds it is bordered by the parasphenoid. It encloses or contributes to the foramina for the cranial nerves V_1,2,3 and VII, all of them exiting through separate foramina. The prootic also contributes to the anterior articulation of the hyomandibular and to the articulatory groove that extends to the pterotic. It also forms a canal for the jugular vein bridged laterally by a narrow ring of bone, the lateral commissure, situated ventrolateral to the anterior articulation of the hyomandibular with the ear capsule. This lateral commissure represents the anterior border of the foramen for the hyomandibular branch of the facialis nerve. The prootic also houses the anterior part of the auditory bulla.

The parasphenoid spans from the orbital to the occipital region (fig. 2B, C). It is narrow in the orbital region, broadest at the level of the anterior articulation of the hyomandibular, then tapers again and splits into two long, thin arms that extend to the articulation of the occipital region with the first vertebra. Anteriorly it has a shallow ventral groove for reception of the vomer, which extends over half of its length. In the area of the trabecula communis in the orbital region, the parasphenoid slightly curves dorsally, becoming detached from the underlying vomer. Thus a narrow gap forms between the two bones.

The pterotic is situated at the widest part of the cranium and forms the posterior part of the articulatory groove and articulation for the hyomandibular (fig. 2B, C). The epiotic is a small cap of bone at the posterodorsal corner of the otic capsule. It may possess a posteriorly directed process in larger specimens to which attach tendons of the trunk musculature.

The broad supraoccipital forms the roof of the posterior part of the cranium (fig. 2A, B). In this large specimen it has a prominent transverse ridge along its posterior margin for insertion of the trunk musculature. There are no parietals. They are not fused to the supraoccipital, but are rather lost, as evidenced by our ontogenetic series of C. keelini.

The exoccipital forms the posterodorsal part of the otic bulla, the dorsal part of the ball and socket articulation with the first vertebra, and the lateral and dorsolateral wall of the foramen magnum (fig. 2). The paired exoccipitals are separated from each other in the dorsal midline by a cartilaginous area. In a dorsal view they may have some small depressions in the bone, which may or may not traverse the bone. Similar depressions are also present in the rostral part of the frontals, on the dorsal face of the nasals and the membranous wings of the lateral ethmoid (fig. 2A). The intercalar is absent. The posteroventral part of the otic bulla and the ventral part of the ball-and-socket articulation with the first vertebra are formed by the basioccipital.

Hyopalatine Arch and Opercular Apparatus (fig. 3):

The hyopalatine arch consists of hyomandibular, symplectic, quadrate, metapterygoid, and ectopterygoid. There is no endopterygoid or palatine (neither dermo- nor autopalatine) developed. The hyomandibular has three articular heads. The anterior head articulates with the autosphenotic/prootic, the posterior head with the pterotic, and the opercular head with the opercle. The crest between the anterior and the posterior head rests in a groove on the prootic and pterotic to which it is attached by connective tissue. The lateral side of the hyomandibular possesses a vertical crest that is bound to the preopercle by connective tissue (fig. 3A). The hyomandibular is pierced by a foramen of the hyomandibular trunk of the facial nerve. An anterior lamina of membrane bone extends toward, but does not contact, the metapterygoid and ventrally contacts the symplectic. The symplectic is long and rodlike and its anterior end fits into a groove in the quadrate (fig. 3B). The latter has a strong posteroventral process of membrane bone tightly bound to the preopercle by connective tissue.

A small zone of cartilage separates the quadrate and metapterygoid (fig. 3). An anterior process of membrane bone from the metapterygoid bridges the gap between this bone and the quadrate. The ectopterygoid is long and narrow and curved like an asymmetrical boomerang. Its posterior end articulates with the medial part of the quadrate. The area of the ectopterygoid that turns from an anterodorsal to a more horizontal plane is tightly bound to the lateral ethmoid. The anteriormost tip of the ectopterygoid is directed toward the midline and there is firmly attached to the vomer.

The four bones of the opercular apparatus are thin and in some areas only weakly ossified (fig. 3A, C). The large opercle articulates with the opercular head of the hyomandibular. It has a prominent ridge on its inner side that extends from the articulation with the hyomandibular caudoventrally (fig. 3C). The subopercle is a thin, weakly ossified plate ventral to the opercle. The preopercle is well ossified in its anterodorsal part, which is connected to the hyomandibular, the symplectic, and the posteroventral process of the quadrate. Ossification is weaker in its posterior area. The interopercle is located medial and ventral to the preopercle; it is wide in the posterior part and attenuates rostrally to a tip (fig. 3C) from which the interoperculo-retroarticular ligament originates. The anterodorsal area of this bone is better ossified than the caudoventral area.

The upper jaw comprises the maxilla and premaxilla, which are tightly bound together (fig. 3B, D). The premaxilla bears teeth along its ventral margin. The maxilla is toothless with a wide posterior part that tapers anteriorly. Its anterior head articulates with the ventrolateral face of the rostral tip of the fused mesethmoid/vomer. The dentary opposes the premaxilla and bears teeth along its dorsal margin. Its posterior end bifurcates into a winglike dorsal coronoid process that is ligamentously attached to the medial side of the posterior end of the maxilla, and a long posteroventral process that runs along the anguloarticular (fig. 3D). The latter bone articulates posteriorly with the quadrate. From its widest part it tapers anteriorly and ends at the base of the bifurcation of the dentary. The posteroventral corner of the anguloarticular is capped by the small retroarticular, to which the ligament from the interopercle is attached. Meckel's cartilage is still well developed and visible in a medial view of the lower jaw. Dorsal to its posterior end lies the small round coronomeckelian bone.

Hyoid, Urohyal, and Branchial Arches (fig. 4):

The interhyal is relatively large (fig. 4A, B). Its upper tip articulates with the hyopalatine arch at the cartilaginous area between hyomandibular and symplectic and its lower tip with the posterior end of the posterior ceratohyal. The latter bone is separated from the anterior ceratohyal by a cartilaginous strip. This strip is bridged on the medial side by a strong caudally directed dorsal process of the anterior ceratohyal that fits into a groove of the posterior ceratohyal (fig. 4B). The anterior ceratohyal has a wide hind part that tapers at about half its length. The dorsal and ventral hypohyals are small ossifications capping the rostral end of the anterior ceratohyal (fig. 4A, B). The ventral hypohyal also serves as point of attachment for the ligament from the urohyal. The dorsal hypohyal articulates with the basihyal and extends posteriorly beyond the posterior tip of the ventral hypohyal. The basihyal is a stout and elongate bone with a cartilaginous tip (fig. 4D).

There are six well-developed long branchiostegal rays, four articulating with the anterior and two with the posterior ceratohyal (fig. 4A, B). The four posterior rays articulate with the lateral face and the anterior ones with the medial face of the ceratohyals. If the branchiostegal membrane is folded, the two anterior rays come to lie medial to the ceratohyals along their whole length and thus are separated from the posterior four rays.

The urohyal (fig. 4C) is bladelike with a vertical lamina of bone and two anterior processes that attach to the ventral hypohyals by ligaments.

There are three ossified basibranchials (fig. 4D), with the first being shorter than the second and third. The latter ends between the third hypobranchials. There are three hypobranchials of different shape. The first hypobranchial is a short rodlike bone. It articulates with the anterior third of the second basibranchial. The second hypobranchial bears a ventrally directed, cartilaginously tipped process at its anterior end. It articulates with the anterior third of the third basibranchial. The third hypobranchial bears several teeth on its dorsal surface. Its anterior end also has a ventrally directed anterior process. The posteriomedial part of hypobranchial 3 is produced into a short broad caudal process with a cartilaginous tip. This articulates with the fourth basibranchial, a small nodule of cartilage that articulates posteriorly with the cartilaginous anterior tip of ceratobranchial 4 (fig. 4D). All ceratobranchials are elongate rodlike bones. Ceratobranchial 5 bears numerous conical teeth on its dorsal surface.

There are four epibranchials whose lateral ends articulate with the respective ceratobranchials (fig. 4D). Epibranchials 1 and 2 are short, thin, and rodlike. The medial end of epibranchial 1 articulates with the anterior end of pharyngobranchial 2; the medial end of epibranchial 2 articulates with the posterior end of the same pharyngobranchial and the anterior end of pharyngobranchial 3. Epibranchial 3 is also rodlike but bears a dorsally directed uncinate process that articulates with a similar process on epibranchial 4. The latter has a club-shaped medial tip for the articulation with the third pharyngobranchial and the fourth upper pharyngeal toothplate (sensu Johnson, 1992). There are only two pharyngobranchials. Pharyngobranchial 2 is the small elongate element whose tips articulate with epibranchials 1 and 2, respectively, and is the anteriormost pharyngobranchial. Its posterior tip is also in contact with the anterior tip of pharyngobranchial 3. The latter is also an elongate bone with a few teeth on the ventral side, indicating its fusion with the respective toothplate. The fourth upper pharyngeal toothplate is roundish in dorsal view with teeth on its ventral side. A pharyngobranchial 4 cartilage is not developed.

Remarks on an Early Developmental Stage of the Cranium:

The cranium of our 7.5-mm larva (fig. 5), the smallest available specimen, is not described in detail but some unusual characters are pointed out. There is a small autogenous cartilage in front of the tip of the ethmoid block, the rostral cartilage, which is still round and knoblike as in other larval and adult percomorphs. The chondrocranium consists of two separate units unconnected by cartilage, the ethmoidal and the otico-occipital units (fig. 5A). The ethmoidal plate has two prominent pairs of processes, long anterior trabecular horns, and laminae orbitonasales at whose lateral tips the lacrimals articulate. The mesethmoid is a long membranous, posterodorsally directed splint of bone on the dorsal aspect of the anterior tip of the nasal septum (fig. 5B). The two large otic capsules are in contact dorsally through the relatively narrow tectum synoticum. The occipital area has a large foramen for the vagal nerve.

The hyosymplectic cartilage is not ossified yet (fig. 5B). The palatoquadrate has no pars autopalatina, and not even the cartilage bar that connects it to pars quadrata et metapterygoidea is developed, as it would be in early stages of many teleosts. Pars quadrata contains an elongate opening. The quadrate's posteroventral process of membrane bone is present. The ectopterygoid is an extremely narrow, needlelike ossification extending from the anterior face of the quadrate to the trabecular horn, and at this stage it does not have the long preorbital extension as in the adult. Meckel's cartilage bears a small anterodorsal process at a position where in many other adult teleosts an ascending process of the anguloarticular is developed. Additional bones present at this stage are: vomer, parasphenoid, frontal, maxilla, praemaxilla, angular, dentary, opercle, preopercle, subopercle, and interopercle. There are no teeth on hypobranchial three in this 7.5-mm specimen, but they are present in our 11.2-mm juvenile.

Shoulder Girdle and Pectoral Fin (fig. 6A):

The cleithrum is the dominant element of the shoulder girdle. It is a stout bone extending from the upper third of the body downward to the midline, where it meets its counterpart in a ligamentous connection. Its dorsal end articulates with the supracleithrum, a small curved bone lateral to it. The cleithrum articulates with two cartilaginous discs that are part of the endoskeletal shoulder girdle but can hardly be referred to as the scapula and coracoid of other fishes. The upper element is the largest and bears a foramen in its dorsal part. There is a weakly developed ossification in its posterior area and an indentation at the upper and lower edge suggesting an incomplete separation into two elements. On the right shoulder girdle this upper element is separated into two plates of cartilage. In other specimens, some variation can be observed regarding the separation of the upper element into two plates. The lower plate is smaller, triangular in shape, and also shows a weak ossification. Its caudal border and the ventral border of the upper element both contact a third mainly cartilaginous element whose middle part is only weakly ossified. The rays of the pectoral fin articulate without intervening distal radials with the caudal edge of the upper element (or, if separate, with the posterior upper plate), and with the lower posterior element that we interpret as possibly a pectoral radial. A postcleithrum is also developed as a thin plate of bone posterior to the cleithrum.

Remarks on the Development of the Shoulder Girdle (fig. 6B):

In our 7.5-mm specimen only the cleithrum is developed as a thin splint of bone. The endoskeletal part of the shoulder girdle is represented by the irregularly shaped curved scapulocoracoid cartilage. The pectoral-fin fold is supported by a broad and thin plate of cartilage, the pectoral radial plate (sensu Johnson and Brothers, 1993), with a central, transverse fissure, which is typical of the developing pectoral fin of many teleosts (see e.g. Potthoff et al., 1984, 1987, 1988; Potthoff and Tellock, 1993; Grandel and Schulte-Merker, 1998; Britz and Johnson, 2002). The pectoral radial plate is still present in a juvenile of 22 mm. In our 37-mm specimen the typical four-part fin skeleton is developed and the fin rays have ossified. The supracleithrum is ossified in a larva of 11.3 mm, and the postcleithrum is present at 22 mm.

Vertebral Column, Dorsal, Anal, and Caudal Fins (figs. 2, 7):

Our figured specimen has 68 vertebrae, the first three vertebrae without ribs (figs. 2, 7A). An epicentral bone (= epineural of Patterson and Johnson, 1995) is attached to the tip of the first transverse process of the left side only (fig. 2), but other specimens have epicentrals on both sides of the first vertebra (see table 1). There are 37 rib-bearing vertebrae starting with the fourth (fig. 7A). There are 28 vertebrae with hemal arches and spines starting with vertebra 40. The first three vertebrae possess prominent lateral parapophyses, which are in a more or less horizontal position (figs. 2, 7A). Towards the more posterior rib-bearing vertebrae, parapophyses are longer and directed ventrally (fig. 7A, B). The first vertebra has a ball-like articular head that fits into a socket on the occipital region of the skull formed by the exoccipitals and the basioccipital (fig. 2). In the ventral midline, the first vertebra has a prominent vertical keel that is connected tightly to the posterior skull base, predominantly to the posterior processes of the parasphenoid (fig. 2). The first six anterior vertebrae bear laterally expanded neural spines (figs. 2, 7A). In lateral view, there is a smaller anterior spinelike projection in front of the expanded neural spine. In more posterior vertebrae this anterior projection gradually becomes less conspicuous and eventually disappears (fig. 7A–C), with vertebra 27 being the last vertebra that still has the anterior projection and the neural spine present. The anterior vertebrae have well-developed pre- and postzygapophyses (fig. 7A–C) that are less prominent toward the tail (fig. 7D) and are eventually absent from the posteriormost vertebrae (fig. 7E). There is also a change in the shape of the vertebrae from the head to the tail region, with the centra becoming much narrower and more elongate with much shorter neural and hemal spines (compare fig. 7A–C with D, E). Another modification along the vertebral column concerns the development of paired fenestrations in the neural arches at the base of the neural spine from about vertebra 5 and posteriorly (fig. 7A, B). These fenestrae become larger in more caudal vertebra so that only a narrow bony bridge connects the prezygapohyses and the anterior spinelike projection dorsally with the neural spine (fig. 7B). Eventually from about vertebra 28 and posteriorly, this bridge is lacking so that the prezygapophses have no dorsal connection to the neural spine (fig. 7C–E). The skeletal support of the caudal fin consists of a single hypural fused to the last centrum (fig. 7E). The caudal fin bears 5 fin rays and is confluent with the dorsal and anal fins.

The dorsal fin has 44 fin rays and 45 pterygiophores, with the first one being somewhat rudimentary and the last one without a serially associated ray. The series of dorsal pterygiophores starts behind the neural spine of vertebra 23. The proximal-middle radials of the pterygiophores show the common upside-down L-shape, the ventral and caudal tips being cartilaginous (fig. 7C–E). In the anterior region of the dorsal series of pterygiophores, they form an angle of about 45° with the vertebral column (fig. 7C), with this angle becoming gradually smaller toward the caudal fin (fig. 7D), so that the last few pterygiophores have an almost parallel orientation to the vertebral column (fig. 7E). There is a small kidney-shaped distal radial intervening between each proximal-middle radial and the bases of each fin ray. It is completely covered by the fin-ray base and cannot be seen in lateral view.

The anal fin has 42 rays and 43 pterygiophores, with the first being rudimentary and the last lacking a serially associated ray. There are 17 pterygiophores in front of the first hemal spine, so that the body cavity reaches much farther posteriorly into the area of the anal fin when compared to other chaudhuriids. Each pterygiophore consists of an L-shaped proximal-middle radial and a tiny kidney-shaped distal radial as in the dorsal fin, except the last one consisting only of the proximal-middle radial. From anterior to posterior, pterygiophores show the same changes in shape and position as described for those of the dorsal fin (see fig. 7C–E).

Information on the intraspecific ranges of several meristic parameters is provided in table 1.

Remarks on Ontogeny of Vertebral Column, Fins, and Fin Supports (fig. 8):

The smallest specimen available, 7.5 mm, has 42 ossified centra (fig. 8A). Cartilaginous neural arches are present on vertebrae 1–33. Left and right halves are separated in the midline by a gap. There are neural spines of membrane bone on the neural arches of vertebrae 1–10. They also fail to meet in the dorsal midline. The first five vertebrae already possess bony parapophyses; they originate at the base of the neural arches in vertebrae 1–4 and on the centrum in vertebra 5. There are seven dorsal-fin pterygiophores developed, with the first situated between neural spines of vertebrae 23 and 24 (fig. 8A). Judging from the position in relation to the vertebrae, the first pterygiophore correlates with the anteriormost pterygiophore in the adult fish so that dorsal-fin pterygiophores apparently develop rostrocaudally. The single hypural of the caudal fin is already chondrified and articulates with a single caudal-fin ray, although the ural centrum has not ossified yet (fig. 8A). There is still a large gap between the hypural and the posteriormost ossified centra.

In the 11.9-mm specimen (fig. 8B), 68 vertebrae are ossified. Vertebrae 1 to about 40 have ossified neural arches, spines, and parapophyses. Posterior to about vertebra 43, neural arches are still in cartilage, as are the hemal arches. Cartilaginous ribs are present on vertebrae 13–37 with the nine posterior ribs being longer than the anterior ones (fig. 8B). Ribs thus seem to develop caudorostrally (they are absent also in an 11.3-mm larva and developed up to vertebra 5 in a 14.5-mm specimen). The caudal fin has no ossified centrum yet; only the cartilaginous hypural, which bears four fin rays, is present. The dorsal fin has 36 cartilaginous proximal radials of which numbers 1–17 possess serially associated distal radials too and numbers 1–29 possess serially associated ossified fin rays (fig. 8B). The anal fin shows a similar condition. There are 36 cartilaginous proximal radials—numbers 1–19 have serially associated distal radials and numbers 1–28 have serially associated ossified fin rays.

The dorsal bony bridges that connect the prezygapophyses with the neural spines on vertebrae 5–27 in the adult specimens are not present yet in our small specimens up to 22 mm. At that stage the neural arches show no fenestration but resemble those of the posterior caudal region of the adult fish. The bony bridges develop later in ontogeny and occur up to vertebra 18 in our 31-mm specimen.

Scales:

Numerous thin cycloid scales (fig. 9) are confined to the posterior part of the body. They are weakly ossified and tiny, measuring 0.3–0.35 mm in diameter. They bear up to six radii in their anterior field and up to 12 concentric circuli on the body of the scale.

Neurocranium (fig. 10):

The shape of the neurocranium in dorsal view differs between the two specimens studied (compare fig. 10A and B). That of the 50-mm specimen has a more elongate shape, tapering continuously from the otic region (the widest part of the cranium) to the ethmoid region (fig. 10B). The 38.2-mm specimen instead shows a slighter attenuation from the otic to the orbital region but then becomes much narrower in the latter part (fig. 10A).

The ethmoidal region consists of the same bones as in C. keelini and differs from that species only in the shape of the different elements. The nasal is smaller and less ossified so that its boundaries are more difficult to discern (fig. 10). It seems to be narrower than in C. keelini and does not reach the mesethmoid in the midline. The lateral ethmoid shows the same anterior dorsal lamina of membrane bone that along with the nasal forms a dorsal wall for the olfactory organ. In the 38.2-mm specimen the nasal and this lamina are fused (fig. 10A, C), but both are clearly separate in the 50-mm specimen (fig. 10B). Contrary to the situation in C. keelini, the lacrimal does not contact the anterior dorsal lamina of the lateral ethmoid or the nasal bone in front of the posterior nasal opening (fig. 10C). The ascending process at the caudal end of the lacrimal is present but does not reach as far dorsally as in C. keelini. There is also a cartilaginous rod, part of the narrow elongate nasal septum, in the midline in front of the lateral ethmoids, which reaches anteriorly to the mesethmoid ossification (fig. 10C), but it appears longer than in C. keelini (fig. 2B).

The vomer extends posteriorly to slightly more than half the length of the neurocranium (fig. 10D). At the border of ethmoidal and orbital region the vomer is separated from the dorsally curving parasphenoid (fig. 10C). This is also present in C. keelini, but is less conspicuous (fig. 2B). The anterior tips of the frontals leave a gap between each other in which the posterior tip of the mesethmoid is located (fig. 10A, B). This gap is wider and more prominent than in C. keelini. The ventral lamina of the frontal in this area is less conspicuous than in C. keelini. Unlike C. keelini, the anterior tip of the large prootic fails to contact the frontal but rather ends freely in the orbital region (fig. 10C, D). Cranial nerves V_2,3 and VII have their openings in the suture between the autosphenotic and prootic (fig. 10C, D). Both bones do not reach anteriorly far enough to enclose the exit of V₁ from the braincase. Parietals are lacking (fig. 10A, B) as in C. keelini. The parasphenoid has a different shape than in C. keelini. Its anterior part is narrower and widens suddenly at the level of the anterior tips of prootic and autosphenotic but narrows again abruptly at the level of the anterior articulation of the hyomandibular (fig. 10D). Its caudal end bifurcates but does not extend beyond the basioccipital. The other bones of the occipital region are similar to the condition described for C. keelini except that the exoccipitals are clearly separated by the supraoccipital (fig. 10A, B).

Hyopalatine Arch and Opercular Apparatus (fig. 11):

The principal arrangement and shape of the different bones that comprise the hyopalatine arch and lower jaw are similar to those in C. keelini. There are a few differences between the two specimens of C. lubricus, however. In the 38.2-mm specimen there is no dorsal lamina of membrane bone of the metapterygoid bridging the cartilaginous strip between this bone and the quadrate (fig. 11 A, B), but there is a short lamina present in the 50-mm specimen (not illustrated). The basal part of the ectopterygoid in both specimens is comparatively broader than in C. keelini. The bones of the opercular apparatus are less stained than in C. keelini. Their distal parts in particular picked up almost no stain and their precise shape and extension had to be studied by transferring them to ethanol and observing them in front of a black background.

Hyoid, Urohyal, and Branchial Arches (fig. 12):

There are no striking differences in the arrangement of the bones of the hyoid. The branchiostegal rays show the same pattern of articulation with the ceratohyals as in C. keelini, that is, the anterior two articulate on the medial face of the anterior ceratohyal, the posterior four on the lateral face of both anterior and posterior ceratohyal (fig. 12A, B). The urohyal is much shallower (fig. 12C), lacking the vertical lamina that characterizes the urohyal of C. keelini.

The most obvious difference between C. lubricus and C. keelini in the branchial arches is the lack of teeth on hypobranchial 3 in both specimens of the former (fig. 12D). The prominent anteroventral process on hypobranchial 2 in C. keelini is lacking in C. lubricus.

Shoulder Girdle and Pectoral Fin (fig. 12E):

This character complex is less developed and ossified compared to C. keelini. Both dermal bones, the cleithrum and the supracleithrum are narrower. There is also a postcleithrum present but it is weakly stained. The endoskeletal support of the pectoral fin is represented by only three separate pieces of cartilage, a larger upper one and two smaller ones below it (fig. 12E). They probably correspond to the three elements reported in C. keelini. No ossification of these elements could be detected in the 38.2-mm specimen (fig. 12E), but a slight pinkish stain in the 50-mm specimen may be evidence of light ossification. There may also have been some slight decalcification of thin bony tissue during the alcian blue staining step because both specimens are double stained.

Vertebral Column, Dorsal, Anal, and Caudal Fins (fig. 13):

The vertebral column of C. lubricus consists of 69 vertebrae. Ribs are developed on 30 or 31 vertebrae, starting with no. 4 or 5, respectively. They are much shorter and smaller compared to those of C. keelini and hardly reach the length of the parapophyses with which they articulate (fig. 13B, C). They are least developed in the middle part between about vertebrae 15 and 30 (fig. 13B) and seem to be missing on some of these vertebrae in contrast to the situation in C. keelini. There are no epicentral bones in the two available specimens (fig. 13A). As in C. keelini, the anterior tip of the first vertebra is modified into a ball-like process for articulation with the socket in the occipital region of the skull (fig. 13A). In the ventral midline it also has a keel-like process (fig. 13A), which is less prominent than in C. keelini. The neural spines of the anterior three (small specimen, fig. 13A) to five (large specimen) vertebrae are expanded. The anterior additional neural spinelike projections of the first few vertebrae in C. keelini are lacking altogether in C. lubricus (fig. 13A). Compared to C. keelini, the characteristic fenestrae at the base of the neural spines are much smaller and less conspicuous in C. lubricus (fig. 13B). They are developed on vertebrae 5–29 in the50-mm specimen (fig. 13B), but only on vertebrae 4–9 in the 38.2-mm specimen. All subsequent vertebrae lack the dorsomedian bony bridge between the parapophyses and the neural spine (fig. 13C). Hemal spines are present on 35 vertebrae starting with vertebra 35. The dorsal fin of the 50-mm specimen consists of 38 pterygiophores, of which the first 37 are serially associated with fin rays, and the last one is not. In the 38.2-mm specimen there are 36 pterygiophores and fin rays, with the first pterygiophore bearing a supernumerary ray in addition to its serially associated ray and the last one lacking a serially associated ray. The dorsal fin starts between vertebrae 31 and 32 (50-mm specimen) or 32 and 33 (38.2-mm specimen) respectively. There are 38 anal fin pterygiophores in the 50-mm specimen, with the first 37 being serially associated with fin rays and the last one lacking a serially associated ray. The 38.2-mm specimen has 37 anal-fin pterygiophores, with the first 36 being serially associated with rays and the last lacking a serially associated ray. The first proximal-middle radial in both specimens is rudimentary. Distal radials occur on all ray-bearing pterygiophores of the dorsal and anal fins. The caudal fin consists of six rays and is confluent with the anal and dorsal fins (fig. 13D). Its rays are supported by a single hypural. In both specimens, the last anal fin ray and distal radial, being serially associated with the penultimate proximal middle radial, are very short and closely approach the terminal centrum with the hypural plate (fig. 13D). This may have caused Kottelat and Lim's (1994: 187) erroneous description of C. lubricus as having seven caudal-fin rays. Meristic data for the two specimens of C. lubricus are summarized in table 2.

There are no scales.

Neurocranium (fig. 14):

The shape of the neurocranium in the larger male P. indica is conspicuously more elongate than in C. keelini, especially in the orbital region but also in the ethmoid region (fig. 14A). This is due to a stronger attenuation of the whole preotic area of the skull. The frontals are apparently fused in the dorsal midline in the larger specimen (fig. 14A), but traces of a suture are visible in the smaller one. As in the two species of Chendol, the lateral ethmoid possesses an anterior dorsal lamina of membrane bone that roofs the posterior part of the olfactory chamber (fig. 14A, B). In ventral view, the lateral ethmoid bears long anterior processes that reach alongside the vomer and mesethmoid to about half of their preorbital length (fig. 14C). The nasal is broad, well developed, and posteriorly overlaps the anterior tip of the fused frontals (fig. 14A). Parietals are lacking. The large supraoccipital bears a pair of curved longitudinal ridges that extend to the posteriormost part of the occiput where they are continued by the epiotic and exoccipital (fig. 14A, B). The supraoccipital separates the exoccipitals in the dorsal midline. The autosphenotic forms most of the lateral wall of the orbital region (fig. 14B). It has a strong lateral process for the origin of the levator arcus palatini. The prootic bears only a short anterior process (fig. 14B, C) and thus differs substantially from the condition in other chaudhuriids except Nagaichthys. The parasphenoid is broad over most of its orbital part and tapers less anteriorly (fig. 14C).

Hyopalatine Arch and Opercular Apparatus (fig. 15):

The most obvious difference from all other chaudhuriids, except Nagaichthys (see fig. 23), in the hyopalatine arch is the presence of only one element in the upper jaw, which has been interpreted in the past as a fused maxilla and premaxilla by Yazdani (1978) and Travers (1984a). The element is a strong, curved bone with numerous teeth on its ventral face. As there are no data on its ontogeny, its identity cannot be resolved at the moment. The lower jaw is also quite massive, and especially in the large male it curves out laterally in its middle part (fig. 15). The ectopterygoid is comparatively weakly developed. As in the two Chendol species, the metapterygoid has a dorsal membranous anterior process that bridges the cartilage strip between this bone and the quadrate. All remaining bones of the hyopalatine arch differ very little from the condition in Chendol.

Hyoid, Urohyal, and Branchial Arches (fig. 16):

The hyoid closely resembles those of other chaudhuriids except that the strong dorsal posterior process of the anterior ceratohyal also covers the dorsolateral face of the posterior ceratohyal and thus can be seen in a lateral view (fig. 16A, B). The urohyal is elongate and its dorsal lamina is comparatively low (fig. 16C).

The overall arrangement of the branchial arches of P. indica shows a close resemblance to that of Chendol. The following differences, however, can be noted. Basibranchial 3 reaches posteriorly between the anterior tips of ceratobranchials 4 in the larger specimen (fig. 16D), but only to the level of the third hypobranchials in the smaller one. There are no teeth on hypobranchial 3, as in Chendol lubricus. Pharyngobranchial 2 is a short stout bone (fig. 16D) and not as elongate as in the other species.

Shoulder Girdle and Pectoral Fin (fig. 16E):

The dermal part of the shoulder girdle does not differ in any essential features from that of Chendol. Its endoskeletal part, however, consists only of two ovoid cartilages. Only the upper one shows some ossification and is connected to the cleithrum. Nine pectoral-fin rays are developed in the larger specimen (fig. 16E), but only six in the smaller specimen.

Vertebral Column, Dorsal, Anal, and Caudal Fin (fig. 17):

The two specimens differ slightly in their number of vertebrae. The 85.8-mm male has 67 vertebrae, the 61-mm female 64. The anteriormost vertebra shows the typical ball-and-socket articulation with exoccipitals and basioccipital (fig. 14). It also bears a ventromedian keel of bone. The parapophysis of the first vertebra is strong and its tip articulates with an epicentral bone (fig. 14). The first three vertebrae have an expanded neural spine (figs. 14, 17A). The additional anterior neural-spinelike process found in C. keelini is not developed in P. indica. Only vertebrae 4–9 show an inconspicuous additional anterior extension (fig. 17A) resembling the anterior spine in C. keelini. There are 24 (23 in the female) rib-bearing vertebrae starting with vertebra 4 (fig. 17A). The parapophyses of the 11 anterior vertebrae end in a wide vertical flange. Ribs of these vertebrae are attached to the upper corner of these flanges (fig. 17A). In more posterior rib-bearing vertebrae, in which parapophyses end in a ventrolaterally directed tip, ribs are attached toward the end of these tips (fig. 17B). In more posterior abdominal vertebrae the parapophyses become longer and are directed more ventrally; the last abdominal vertebra of the 85.8-mm specimen, which is vertebra 28, has very long parapophyses, not developed in the 61-mm specimen. In both specimens, this vertebra bears no ribs. The first caudal vertebra that is the first vertebra with the hemal arches connected in the ventral midline is vertebra 29 in the 85.8-mm specimen and 28 in the 61-mm specimen.

From vertebra five on, the fenestration of the neural arch is developed at the base of the neural spine (fig. 17A). These fenestrae become comparatively larger toward more posterior vertebrae (fig. 17B, C), so that the narrow bony bridge, already described above in the two Chendol species, is present in the dorsal midline between the neural arch and the prezygapophyses of the respective vertebrae. Eventually, this bony bridge is absent from all vertebrae posterior to vertebra 39 in the 85.8-mm specimen and vertebra 41 in the 61-mm specimen so that their prezygapophyses and the neural spine are no longer connected dorsally (fig. 17D).

In the 85.8-mm specimen the dorsal fin has 39 pterygiophores and fin rays, with the first pterygiophore bearing a supernumerary ray and the last one without a serially associated ray. The 61-mm specimen has 35 pterygiophores and rays; the first pterygiophore has a supernumerary ray and the last is without a serially associated ray. The dorsal fin starts in both specimens between vertebrae 27 and 28. In the 85.8-mm specimen there are 39 pterygiophores in the anal fin, of which the anterior 38 support serially associated rays and the last lacks a serially associated ray. The 61-mm specimen has 36 pterygiophores and fin rays, with the first pterygiophore supporting a supernumerary ray and the last lacking a serially associated ray. There are three pterygiophores in front of the first hemal spine in the 85.8-mm specimen and two in the 61-mm specimen; the anteriormost pterygiophore of the larger specimen is rudimentary. The last dorsal pterygiophore is situated posterior to the neural spine of the first preural vertebra in the 85.8-mm specimen, but anterior to the neural spine of the second preural vertebra in the other. The last anal pterygiophore sits anterior to the hemal spine of the first preural vertebra in both specimens (fig. 17D). Thus, the dorsal and anal fins are confluent with the caudal. The two hypurals are fused to the ural centrum and each supports five caudal-fin rays (fig. 17D). Meristic data for the two specimens of P. indica are summarized in table 3.

There are no scales.

Neurocranium (fig. 18):

The overall shape of the neurocranium resembles roughly that of the two species of Chendol. The following differences are noted. The nasal and lacrimal are poorly ossified at their anterior tips. The lacrimal is much less developed and narrower. Its posterior tip articulates with a prominent cartilaginous knob that arises from the ventral part of the wings of the lateral ethmoid (fig. 18B, C). The lacrimal has no ascending process at its posterior end and thus fails to contact the lateral ethmoid. The latter bone is only a small ossification building the caudal wall of the nasal capsule but lacks an anterior dorsal lamina and any contact with the frontal. The foramen for V₃ of the trigeminal nerve is located exclusively in the prootic bone (fig. 18B, C) and not in the sutural line between prootic and autosphenotic. The most obvious difference to the Chendol species and P. indica is the presence of a parietal in the skull roof (fig. 18A, B). It is an ovoid bone overlying the lateral part of the supraoccipital, the caudal part of the frontal, the anterior part of the epiotic, and the medial area of the pterotic. Both exoccipitals meet in the dorsal midline and thus keep the supraoccipital from bordering the foramen magnum (fig. 18A).

Hyopalatine Arch and Opercular Apparatus (fig. 19):

This complex shows principally the same arrangement as in the two species of Chendol. There is a prominent anterior dorsal process on the metapterygoid that bridges the cartilaginous strip between this bone and the quadrate. The ectopterygoid has a much wider proximal portion articulating with the medial face of the quadrate (fig. 19B). This part of the ectopterygoid also shows a short but prominent projection at its posterior face. Another difference is the presence of a short distinct process on the posterior part of the anguloarticular from which a strong ligament originates extending to the posterior arm of the maxilla.

Hyoid, Urohyal, and Branchial Arches (fig. 20):

These elements are not significantly different in shape or arrangement from the condition in Chendol or Pillaia. The urohyal (fig. 20C) has a shallower vertical lamina than C. keelini, but a comparatively deeper one than in C. lubricus or P. indica.

The branchial arches (fig. 20D) show a similar structure as in the Chendol species and P. indica. There are several teeth on hypobranchial 3 as in C. keelini, but unlike P. indica. A difference, however, is the relative size of pharyngobranchial three with its fused toothplate to the fourth upper toothplate behind it. In Chendol species and in P. indica, pharyngobranchial 3 is smaller than the fourth toothplate (figs. 4D, 12D, 16D), but in C. caudata the former is larger, almost double the size of the latter (fig. 20D). Hypobranchial 2 has an anteroventrally directed process, as in C. keelini.

Shoulder Girdle and Pectoral Fin (fig. 20E):

The three exoskeletal parts of the shoulder girdle, the cleithrum, supracleithrum, and postcleithrum, show a similar structure as in Chendol and Pillaia. There are, however, differences in the endoskeletal part. There is only an undivided block of cartilage present with two weak ossifications (fig. 20E)—one at the dorsal area and one at the ventral area separated by a broad zone of cartilage. The upper ossification is pierced by a foramen. This condition is difficult to compare with either the adult condition of C. keelini in which three or four pieces are present (fig. 6A), that in C. lubricus with its three unossified parts (fig. 12E), or that in P. indica, which has two cartilages (fig. 16E).

Vertebral Column, Dorsal, Anal, and Caudal Fins (fig. 21):

The figured Chaudhuria caudata has 73 vertebrae, more than any of the Chendol species or P. indica, although their number varies according to the sample (see table 4). The first vertebra bears a strong medioventral keel (fig. 18B), as in other chaudhuriids, and its parapophysis supports a stout epicentral bone (fig. 18). The first four vertebrae possess an expanded neural spine (figs. 18B, 21A). From vertebra four on, there is an anterior spinelike projection in front of the actual neural spine (fig. 21A). This projection becomes comparatively shorter toward posterior vertebrae (fig. 21B, C) and is eventually lacking on vertebrae of the posterior caudal region (fig. 21D, E). The characteristic fenestration of the neural arches of anterior vertebrae, present in Chendol and Pillaia, is lacking in this specimen of Chaudhuria (fig. 21A–E, but see below for Chaudhuria from other localities). There are 24 vertebrae bearing well-developed ribs starting with vertebra 4 (fig. 21A). The first vertebra with a hemal spine that is also the first one without a rib is vertebra 28. There are in total 46 vertebrae with hemal spines. From vertebra 48 on, hemal arches develop wide lateral expansions distally (fig. 21C), which in more posterior vertebrae (from no. 49 on) contact and fuse to a lamina of bone that extends from the lower face of the centrum ventrally, resulting in a cylindrical hemal arch with a very broad base in lateral view (fig. 21D). Toward the caudal fin the neural and hemal spines become shorter so that the last 10 vertebrae appear to lack them completely (fig. 21E). The dorsal fin has 43 pterygiophores and 42 rays starting between vertebrae nos. 26 and 27. The first 42 pterygiophores are serially associated with rays, but the last one lacks a serially associated ray. The anal fin consists of 44 pterygiophores, with three in front of the first hemal spine and 41 behind it. The first pterygiophore bears a supernumerary ray and the last one lacks a serially associated ray. In contrast to the two Chendol species and P. indica, the caudal fin is separated from the dorsal and anal fin by a gap of four vertebrae. Two hypurals support eight caudal-fin rays (fig. 21E).

Differences among CHAUDHURIA from Different Localities (for meristic differences refer to table 4): Chaudhuria caudata from Malaysia, Terengganu (CMK 8241): All six specimens from this lot stained poorly with alizarin. However, a few characters could still be noted. The exoccipitals are separate in four specimens and almost meet in two. In four specimens there is an epicentral bone on both sides, but in two specimens it is developed on the right side only. There are no fenestrations on the neural arches in any of the specimens, as in CMK 7934. Unusually, the hemal spines of the first 12–18 vertebrae posterior to the last rib-bearing vertebra fail to meet in the ventral midline.

Chaudhuria caudata from Viet Nam, Phu Quoc (AMNH 217415): The shape of the postorbital skull appears to be shorter and wider than in CMK 7934. Exoccipitals meet in the dorsal midline. There is no fenestration of the neural arches. Vertebra 43 is the first vertebra with an expanded hemal arch, and vertebra 49 is the first with this expansion being continuous with the ventrolateral face of its centrum. Neural and hemal spines of the caudal vertebrae become smaller toward the caudal fin but are still discernible. The caudal fin bears seven rays, four articulating with the upper and three with the lower hypural.

Chaudhuria caudata from Laos, Vientiane Province (CMK 15965): There are no epicentral bones in any of the six specimens. Exoccipitals approach each other closely in the dorsal midline in five and actually contact each other in one specimen. Contrary to C. caudata from Thailand and Viet Nam, in most specimens the neural arches of the posterior abdominal and anterior caudal region possess paired fenestrae in the neural arches. On the more caudal vertebrae there are no lateral extensions of the middle part of hemal arches, as those found in CMK 7934 or AMNH 217415. Rather, the bases of the hemal arches are in broad contact with the centra. The first preural centra bear well-developed neural and hemal arches (fig. 21F) and do not have the cylindrical shape as those of CMK 7934 (fig. 21E).

Remarks on the Osteology of CHAUDHURIA FUSIPINNIS (CMK 15967): This species has been described only recently (Kottelat and Britz in Kottelat, 2000). Apart from the characters provided in the original description and listed below, there are no significant differences from C. caudata (CMK 7934) in the overall shape or arrangement of the skeletal osteology that would warrant a more detailed osteological account of this species.

Chaudhuria fusipinnis differs from C. caudata in having the dorsal and anal fins confluent with the caudal fin (fig. 21G), so that the posteriormost pterygiophores of the dorsal and anal fin are located in front of the neural and hemal spines of PU2, respectively (in other Chaudhuria, in front of PU5, 6, or 7). This results in a comparatively higher number of dorsal and anal-fin rays. The species is further distinguished by the lack of teeth on hypobranchial three and by only 3+3 caudal-fin rays (fig. 21G). In our six specimens, the anterior five to seven neural spines are expanded. There are no epicentral bones on the first vertebra. In contrast to C. caudata from Thailand or Viet Nam, C. fusipinnis possesses the paired fenestrae in the neural arch found in other chaudhuriids. Depending on the specimen, fenestration starts already on anterior abdominal vertebrae or is restricted to caudal vertebrae. Additional meristic information is provided in table 4.

Neurocranium (fig. 22):

The overall shape of the neurocranium resembles that of Chendol and Chaudhuria with the exception that in Nagaichthys the skull does not taper anteriorly to the same extent as in the other two genera. This makes the frontals almost rectangular in dorsal view (fig. 22A). A further difference is the prootic's anterior extension of membrane bone, which is very short and hardly reaches half the length of the autosphenotic (fig. 22B, C). This condition resembles that in P. indica (fig. 14B, C). Thus it encloses the foramina for the hyomandibular trunk of VII and the mandibular branch of V (fig. 22B, C), but not its maxillary and ophthalmic branches. A third major difference is the well-developed supraoccipital crest in Nagaichthys, which is lacking in the other taxa. The supraoccipital clearly separates the exoccipitals in the dorsal midline (fig. 22A). As in Chaudhuria and Bihunichthys, Nagaichthys has a parietal, which is lacking in Chendol and Pillaia.

Hyopalatine Arch and Opercular Apparatus (fig. 23):

The hyopalatine arch also differs in several aspects from that of the other species described above. The ectopterygoid is reduced in size (fig. 23), even more so than in P. indica (fig. 15). The anterior arm that runs parallel to the vomer is especially short and thin. It ends just anterior to the lateral ethmoid and unlike in Chendol, Pillaia, or Chaudhuria, does not reach farther anteriorly. Contrary to what was reported in the original description (Kottelat and Lim in Kottelat, 1991), the C&S paratype of Nagaichthys filipes has the maxilla and premaxilla fused over most of their length (fig. 23), although the original shape of the bones is still discernible. The anterior process of the metapterygoid is present, though rudimentary, and does not bridge the cartilaginous strip between metapterygoid and quadrate.

Hyoid, Urohyal, and Branchial Arches (fig. 24):

The urohyal is relatively small and has a comparatively high vertical lamina (fig. 24C), but the rest of the hyoid arch is similar to that of the described species. The branchial arches have several reductive characters (fig. 24D): there are no teeth on hypobranchial 3, and the anteroventral process on hypobranchial 2 is lacking. Pharyngobranchial 3 and the fourth upper pharyngeal toothplate have a similar size.

Shoulder Girdle and Pectoral Fin (fig. 24E):

Other reductive differences distinguishing N. filipes from Chendol, Pillaia, and Chaudhuria concern the shoulder girdle and pectoral fin. The supracleithrum is tiny (fig. 24E). The endoskeletal part of the pectoral-fin support consists of a single cartilaginous piece bifurcating proximally at its articulation with the cleithrum. There are three zones slightly stained, which represent weakly ossified areas. There is typically only a single segmented pectoral-fin ray articulating with that piece of cartilage (but see tables 5, 6 for variation). The postcleithrum seems to be absent, although there is a possibility that it is only poorly ossified and did not stain with alizarin.

Vertebral Column, Dorsal, Anal, and Caudal Fins (fig. 25):

The C&S paratype of N. filipes shows a formerly damaged (bitten off?) and later regenerated caudal fin, as already remarked by Kottelat and Lim (in Kottelat, 1991). The total 54 vertebrae may be slightly low for this species, due to the regenerated caudal region. No epicentral bones are developed. The neural spines of the first two vertebrae are expanded (fig. 25A). There are 23 rib-bearing vertebrae starting with no. 4. Ribs are comparatively short and thin (fig. 25A, B), only one-fourth to one-third the length of the supporting parapophyses, resembling the condition in C. lubricus (fig. 13B, C). All 26 abdominal vertebrae show some fenestration of their neural arches (fig. 25A, B). It consists of numerous small foramina on the first seven, but of a large paired foramen on subsequent vertebrae up to 26. As in Chendol species, P. indica, and some Chaudhuria, a bony bridge that extends between the prezygapophyses and the neural spine forms the dorsal border of the foramen (fig. 25B). This bony bridge is absent from all 27 vertebrae with hemal spines (fig. 25C).

The dorsal fin has 29 fin rays and pterygiophores and starts between vertebrae 25 and 26. The anal fin also has 29 fin rays and pterygiophores, with the anterior 28 pterygiophores being serially associated with fin rays and the first pterygiophore supporting a supernumerary ray. There are two pterygiophores in front of the first hemal spine, and 27 behind it. Due to the regenerated caudal area of our specimen of N. filipes, this number of dorsal- and anal-fin pterygiophores and rays may be too low (see tables 5, 6 for variation among samples). Nothing definite can be said about the caudal fin of this specimen of N. filipes. The last vertebra, no. 54, does not articulate with any fin rays. Caudal to it are several cartilages, modified pterygiophores of the dorsal and anal fin that support seven fin rays thus forming a “new”, functional caudal fin.

There are, however, an additional four C&S specimens of Nagaichthys available for study from localities other than the type locality. They agree in all essential details with the description provided above, but, because their caudal fins are undamaged, they provide information on its precise structure. We describe here the caudal fin of the 28-mm specimen of CMK 10870 (from Sarawak, fig. 25D). Dorsal, anal, and caudal fins are confluent. The latter consists of a single hypural fused to the ural centrum and four caudal-fin rays articulating with it. The caudal-fin area is peculiar in that the last dorsal- and anal-fin rays are much shorter than either the penultimate fin ray of dorsal or anal, or the dorsal- or ventralmost caudal-fin ray, resulting in a frayed appearance (fig. 25D, arrowheads). This special arrangement of the caudal-, dorsal-, and anal-fin rays was found in all additional specimens of N. filipes.

Differences among the samples of N. filipes from various localities in the number of vertebrae and fin rays are provided below in tables 5 and 6. All the alcohol specimens we studied had a number of free neuromasts on the head, although their presence was not specifically mentioned in the original description of N. filipes (Kottelat and Lim in Kottelat, 1991).

Neurocranium (fig. 26):

In dorsal view the shape of the neurocranium does not have the strongly attenuated orbital area of Chendol (figs. 2, 10), Pillaia (fig. 14), and Chaudhuria (fig. 18), but is rather fairly rectangular (fig. 26A), resembling more the condition in Nagaichthys (fig. 22). Bihunichthys differs from Chendol species and P. indica in the lack of an anterior dorsal lamina on the lateral ethmoid and the presence of a parietal (fig. 26A, B). It differs from all other chaudhuriids in that the anterior membrane bone extensions of the autosphenotic and prootic are narrow, needlelike, and free from each other and from the frontal (fig. 26B). This condition of the two bones, however, is characteristic of smaller juvenile specimens of Chendol keelini. Only the foramina for the mandibular branch of the trigeminal nerve and for the facial nerve are present at the suture between prootic and autosphenotic (fig. 26B, C). The exits for the other two branches of the trigeminal nerve, V₂ and V₃ from the brain cavity, are located farther anteriorly and are thus not enclosed by bone. In the occipital region, the exoccipitals are widely separated in the dorsal midline by the supraoccipital (fig. 26A), as in small specimens of Chendol keelini and in adults of C. lubricus, P. indica, and N. filipes.

Hyopalatine Arch and Opercular Apparatus (fig. 27):

Except for its smaller size, no major differences in the hyopalatine arch and opercular apparatus from other chaudhuriids could be observed. The anterior membrane bone process of the metapterygoid is well developed. The ethmoid part of the ectopterygoid is more slender and the whole bone comparatively small, but not as reduced as in N. filipes (fig. 23).

Hyoid, Urohyal, and Branchial Arches (fig. 28):

The overall arrangement of the bones is similar to that of other chaudhuriids. Hypobranchials 2 and 3 possess anterior processes (fig. 28D). Hypobranchial 3 bears only few teeth in the 42-mm specimen of AMNH 217765, but is toothless in the 30-mm specimen of CMK 7947. As in C. caudata (fig. 20D), the fourth upper pharyngeal toothplate of B. monopteroides (fig. 28D) is comparatively smaller than in the other chaudhuriids (figs. 4D, 12D, 16D, 24D).

Shoulder Girdle and Pectoral Fin (fig. 28E, F):

The main difference from all other chaudhuriids, except some specimens of Nagaichthys, is the presence of only one pectoral-fin ray. It articulates with a roughly rectangular cartilaginous plate of the endoskeletal shoulder girdle that, depending on the specimen, shows some ossification proximally. The remaining part of the endoskeletal shoulder girdle also shows some variation among specimens. In AMNH 217765 (fig. 28E), it consists of two separate plates of cartilage (scapula and coracoid cartilages?), with the upper element being pierced by a foramen. The 36-mm specimen of CMK 7947 (fig. 28F) has only one larger piece of cartilage whose distal two-thirds are ossified and which bears a foramen. In this specimen the supracleithrum is also better developed. As in N. filipes, but unlike other chaudhuriids, the postcleithrum seems to be lacking in all our specimens of B. monopteroides, unless it is too poorly ossified to stain with alizarin.

Vertebral Column, Dorsal, Anal, and Caudal Fins (fig. 29):

The specimen of B. monopteroides figured herein has 74 vertebrae, and together with C. fusipinnis (table 4) has the highest number among chaudhuriids (see also table 7 for variation among B. monopteroides). Only the three anterior vertebrae have expanded neural spines (fig. 26B). They also possess a complete roof above the neural canal without fenestration. On subsequent vertebrae the fenestration of the neural arches with the typical bony bridge dorsal to the paired fenestrae, as described in most other chaudhuriids, is not present, except on one or two individual vertebra (e.g., fig. 29A, vertebra 7; also vertebra 3 of 30-mm specimen of CMK 7947). The vertebrae rather resemble posterior caudal vertebrae of other chaudhuriids or earlier developmental stages of anterior vertebrae in C. keelini, in both of which the dorsal bridge has not formed. Thus, the vertebrae of B. monopteroides, except the most anterior ones, bear a deep incisure in the bony lamella between the prezygapophysis and the neural spine. In the 36-mm specimen of CMK 7947 this type of vertebra starts only at vertebra 18. All preceding vertebrae have a more or less complete bony roof above the neural canal. In the same specimen, the anterior 28 vertebrae possess an anterior neural spinelike process on the centrum, whereas this is only present on vertebrae 3–12 in the figured specimen (fig. 29A) or 4–13 in the 30-mm specimen of CMK 7947. The posteriormost 9–10 caudal vertebrae in all three specimens are long bony cylinders without any neural or hemal spines (fig. 29D). In the specimen figured, short, small ribs are developed on vertebrae 4 through 7 or 8, depending on the side (fig. 29A). Hemal spines are present from vertebra 35 on. There are 40 dorsal-fin pterygiophores starting between vertebrae 23 and 24, with the anterior 39 being serially associated with fin rays and the last lacking a serially associated ray. The anteriormost pterygiophore is rudimentary. The anal fin has 41 pterygiophores with 11 in front of the first hemal spine. The anterior 40 are serially associated with fin rays, the anteriormost pterygiophore bears a supernumerary ray, and the last lacks a serially associated ray. The caudal fin of Bihunichthys (fig. 29D) is peculiar in possessing only a single long caudal-fin ray that articulates with the tip of a single hypural fused to the elongate terminal centrum. Meristic data for the three studied specimens of B. monopteroides are summarized in table 7.