Craniodental form and function were evaluated in the 2 extant, parapatric species of dermopterans (Dermoptera: Cynocephalidae). The level of morphological distinction between the 2 taxa was such that were they known as fossils; most taxonomists would not contest their genus-level distinction as indicative of adaptive differentiation. In fact, these taxa exemplify the widely employed but implicit morphological distance-based standards used for delineating mammalian genera in the paleontological literature. Appropriate names for these 2 taxa are Cynocephalus volans for the Philippine flying lemur and Galeopterus variegatus for the heterogeneous populations of the Sundaic flying lemurs. Cynocephalus probably has a hypertrophied version of the ancestral cynocephalid molar complex and modified incisor and canine morphology. The hypertrophied metaconules of cynocephalids occlude with an expanded paracristid and a cusplike shelf, the distocuspid. Cynocephalus also has a broader rostrum, a greater degree of postorbital constriction, and enhanced ectocranial ridges associated with a more robust masticatory musculature than Galeopterus. Cynocephalus appears adapted to a diet that requires a greater degree of shearing by the anterior dentition and crushing by the molariform dentition. These anterior shearing teeth (I3, C1, P3, c1, p3) are larger and more bladelike than those of Galeopterus, and the bite force is more anteriorly directed. Angle of the mandible is ventrally expanded in Cynocephalus, facilitating enhanced chewing force for the postcanine dentition while maintaining orientation of the temporalis muscle. Dwarfed forms of Galeopterus are found on many of the smaller islands of the Sunda Shelf and in central Laos. They are not morphologically distinguishable from larger members of this species, other than in size, and do not warrant specific distinction. However, it may be desirable to designate 4 subspecies of G. variegatus: G. v. variegatus from Java, G. v. temminckii from Sumatra, G. v. borneanus from Borneo, and G. v. peninsulae from the Malay Peninsula and mainland Southeast Asia. Separate species rank for each of the dwarfed populations should not be recognized. Phylogenetic relationships of Dermoptera are discussed in light of the morphological differences of the 2 genera.