study specimens of 25.0 mm and larger the centra of the 

 first five vertebrae are all fully fused to one another and 

 to the basioccipital, while their neural arches become 

 progressively more fused together until only the lobation 

 dorsally remains from their original distinctness (see 

 Tyler 1963a: 158- 160 for details). After the centra have 

 fused with the basioccipital I am unable to distinguish 

 where the one ends and the other begins. In the adult, the 

 posterior end of the fused basioccipital and first five 

 vertebral centra is a concave rounded surface which ar- 

 ticulates by fibrous tissue with the sixth abdominal 

 vertebra. One can suppose that this concave articular 

 surface is formed from material contributed by the cen- 

 trum of the fifth vertebra. At least the first two vertebrae 

 seem to lie principally lateral to the posterior region of 

 the basioccipital, for it appears that they have fused not 

 to the extreme posterior end of the basioccipital but 

 rather to its posterolateral surface. The neural arch of the 

 first vertebra overlies and interdigitates with the postero- 

 dorsal surface of the exoccipitals. Ventrolaterally from 

 each side of the fused first five vertebral centra there is a 

 wide, but delicate, ventral process which attaches by fi- 

 brous tissue posteriorly with a similar pair of projections 

 from the sixth abdominal vertebra. The small muscles 

 which attach to these ventral projections have a com- 

 plex relationship with the air bladder. The dorsomedial 

 edges of the neural arches of the first five vertebrae meet 

 in the midline and are held together at first by fibrous 

 tissue, but in large specimens this articulation becomes 

 interdigitated to form a complete bony roof over the 

 neural canal. No neural spines are present. The lower 

 half of the posterior edge of the fused neural arches of the 

 first five vertebrae strongly interdigitates with the an- 

 terior edge of the lower portion of the neural arch of the 

 sixth abdominal vertebra. 



Other abdominal vertebrae. —As explained in the 

 preceding section on the Aracanidae, the first caudal 

 vertebra is arbitrarily designated as the anteriormost 

 vertebra whose ventral surface is most closely associated 

 with the proximal end of the first anal fin basal pteryg- 

 iophore which consistently lies in the midvertical plane 

 of the body. In 13 specimens there are nine abdominal 

 vertebrae; which is to say that the first five vertebrae are 

 fused into a single piece, and that this element is fol- 

 lowed by four separate and distinct vertebrae. The sixth 

 abdominal vertebra articulates by fibrous tissue at its 

 concave anterior end with the posterior concave surface 

 of the fused first five vertebrae and basioccipital, while 

 laterally the lower half of the anterior edge of the neural 

 arch of the sixth vertebra is extensively interdigitated 

 with the lower posterior edge of the neural arch of the fus- 

 ed first five vertebrae. The sixth abdominal vertebra 

 possesses a short and thin neural spine along most of the 

 length of the dorsal surface of its neural arch. The 

 haemal processes from the ventrolateral edges of the an- 

 terior end of the sixth vertebra are anteriorly expanded 

 at their distal ends. The distal ends of these processes do 

 not fuse to one another in the midline of the body; rather, 

 they are held to one another by fibrous tissue. This in- 



complete haemal arch is held by fibrous tissue an- 

 teriorly to the similar structure from the fused first five 

 vertebrae and, like the latter, it is a point of attachment 

 for a complex of muscles associated with the air bladder. 

 The seventh to ninth abdominal vertebrae have large 

 neural spines and complete, although much reduced, 

 haemal arches without haemal spines. The sixth to ninth 

 abdominal vertebrae articulate with one another not only 

 by fibrous tissue between their concave articular faces 

 but also by deep and firm interdigitation of the edges of 

 the neural arches. The neural spine of the seventh ab- 

 dominal vertebra is long, thin, and delicate; the antero- 

 posterior depth of its basal region is about equal to that 

 of the vertebra, but the spine tapers to a point distally. 

 The posterior edge of the neural spine of the seventh ab- 

 dominal vertebra articulates by fibrous tissue with the 

 first dorsal fin basal pterygiophore. The neural spines of 

 the eighth and ninth abdominal vertebrae are narrow 

 rods arising from the posterodorsal surfaces of their 

 neural arches and directed posterodorsally between the 

 dorsal fin basal pterygiophores almost to the distal ends 

 of the latter elements. The neural spine of the eighth ab- 

 dominal vertebra lies between the first and second 

 pterygiophores, and the neural spine of the ninth 

 between the third and fourth pterygiophores. The neural 

 spines and pterygiophores are held to one another by a 

 sheet of fibrous tissue. The haemal arches of the seventh 

 to ninth abdominal vertebrae are complete, although 

 thin and very delicate and of somewhat variable shape 

 from specimen to specimen. No haemal spines are pres- 

 ent. The haemal arches arise from the anterior end of the 

 ventrolateral surfaces of the centra and do not project 

 very far ventrally, thus enclosing only a shallow cavity 

 through which the haemal canal runs. The arches of the 

 eighth and ninth vertebrae are distinctly displaced to one 

 side of the midline of the body, usually to the right. The 

 longitudinal concavity on the ventral surface of the 

 seventh to ninth vertebrae, which marks the course of the 

 haemal canal, is likewise displaced to a greater or lesser 

 degree to one side of the midline of the body. The 

 longitudinal concavity on the ventral surface of the sixth 

 vertebra is in the midline of the body, as it is on the an- 

 terior end of the seventh vertebra. As the concavity 

 passes from the anterior to the posterior end of the 

 seventh vertebra, it is displaced usually to the right of 

 the midline, and along the eighth and ninth vertebrae it 

 is similarly usually displaced to the right of the midline. 

 The ventrolateral edges of the seventh to ninth ab- 

 dominal vertebrae are interdigitated with one another. 

 The eighth and ninth abdominal vertebrae possess 

 slightly upraised ridges diagonally across the lateral sur- 

 faces of their centra from the bases of the neural spines to 

 the anterolateral edges of the centra. The neural foramen 

 of each of the abdominal vertebrae from the sixth to 

 ninth emerges relatively low on the lateral surface of the 

 vertebra. 



Caudal Vertebrae. — The caudal vertebrae numbered 

 10 in 13 specimens, although all other species of os- 

 traciids, including the other three species of Acantho- 



