The ontogeny of Belosaepia ungula Gabb, 1860 from the Crockett Formation (Bartonian stage, Eocene) of Texas is documented for growth from embryo to old age. During the last stage of life, much skeletal resorption occurred, resulting in a major change in form of the skeleton. The animal produced a large skeleton (to 180 mm in length and 50 mm in diameter) with endogastric coiling, oblique septa and a very large siphuncle. The skeleton has a guard with a solid posterior prong, a posteroventral corona plate and a noded dorsal shield. The ventral margin of the skeleton consists of a thin flattened deck containing strongly recurved septa, conotheca and a secondary prismatic shell layer. New terms are defined for features of the skeleton not previously described. The microstructure of the ventral deck and the presence of a rod structure between the prong and callus are described for the first time. Chamberlets similar to those in living Sepia cuttlebones are present between closely spaced septa and they vary from walled units on lateral margins to pillar form in mid-ventor. The siphuncle is secondarily thickened within the dorsal interior, producing a siphuncle band. The skeleton was produced by a deep-bodied animal of demersal life habits. The species B. uncinata, B. harrisi and B. alabamensis voltzi proposed by Palmer (1937) are synonymised with B. ungula. The species B. veatchii and B. saccaria of Palmer (1937) are considered to be valid species, but B. alabamensis proposed by Palmer (1937) is synonymised with B. veatchii. Descriptions of belosaepiid species must be based only on specimens of adult size that have not been affected by resorption.