stracion, have only 9 caudal vertebrae (see Tyler 

 1965b:268). The first and second caudal vertebrae are 

 like the eighth and ninth abdominal vertebrae, except 

 that their neural spines are shorter and that their antero- 

 ventral surfaces support by fibrous tissue the first two 

 anal fin basal pterygiophores which lie entirely in the 

 midvertical plane of the body. The neural spine of the 

 first caudal vertebra lies between the fifth and sixth dor- 

 sal fin basal pterygiophores. The haemal arches of the 

 first, second, and third caudal vertebrae are delicate and 

 complete, like those of the seventh to ninth abdominal 

 vertebrae. The haemal arches of the second and third 

 caudal vertebrae are more or less symmetrically placed 

 in the midline of the body, but that of the first caudal 

 vertebra is displaced somewhat to the right. The longi- 

 tudinal concavity present on the ventral surfaces of the 

 first to fourth caudal vertebrae, which marks the course 

 of the haemal canal, is more or less in the midline, ex- 

 cept in the case of the first caudal vertebra, where the 

 concavity is displaced to the right at the anterior end of 

 the vertebra. The fourth caudal vertebra usually 

 possesses only the longitudinal concavity on its ventral 

 surface, and no haemal arch. The third and fourth anal 

 fin basal pterygiophores that lie in the midvertical plane 

 of the body articulate by fibrous tissue with the antero- 

 ventral surfaces of, respectively, the third and fourth 

 caudal vertebrae. The neural spine of the third caudal 

 vertebra is anteroposteriorly expanded into a flattened 

 plate which along the dorsal half of its anterior edge is 

 firmly articulated by fibrous tissue with the posterior 

 edge of the last dorsal fin basal pterygiophore, while 

 along its posterior edge it is deeply interdigitated with 

 the anterior edge of the neural spine of the fourth caudal 

 vertebra. The neural spine of the fourth caudal vertebra 

 is anteroposteriorly expanded into a flat plate which is 

 firmly interdigitated at its anterior and posterior edges 

 with, respectively, the posterior edge of the neural spine 

 of the third caudal vertebra and the anterior edge of the 

 neural spine of the fifth caudal vertebra. Whereas the 

 third caudal vertebra is the last caudal vertebra to make 

 contact with the dorsal fin basal pterygiophores, the fifth 

 caudal vertebra is the last to make contact with the anal 

 fin basal pterygiophores. The fifth caudal vertebra has 

 its neural spine anterodorsally expanded, with its an- 

 terior edge interdigitating with the posterior edge of the 

 neural spine of the fourth caudal vertebra, while its pos- 

 terior edge is well separated fi-om the neural spine of the 

 sixth caudal vertebra and is held to the latter only by fi- 

 brous tissue. The fifth caudal vertebra possesses a well- 

 developed haemal arch and spine, with the haemal arch 

 running the length of the vertebra. The haemal spine of 

 the fifth caudal vertebra is a thick compressed plate 

 which is concave along the lower half of its anterior edge 

 and articulates by fibrous tissue with the middle region 

 of the posterior surface of the last anal fin basal pteryg- 

 iophore. The sixth to ninth caudal vertebrae are basically 

 similar to one another. They possess sturdy neural arches 

 and spines and haemal arches and spines, all of which 

 decrease slightly in size posteriorly in the series. The cen- 

 tra of these four vertebrae are much shortened antero- 



posteriorly and are held to one another by fibrous tissue. 

 These vertebrae, and the last vertebra, are thus the only 

 vertebrae which can be laterally flexed to any ap- 

 preciable extent, since all the vertebrae anterior to the 

 sixth caudal vertebra are interdigitated with one another 

 over large areas of their surfaces of contact. Thus, the 

 third to sixth caudal vertebrae are interdigitated with 

 one another along the surfaces of contact of their neural 

 arches and spines; the seventh abdominal to the second 

 caudal vertebrae are interdigitated with one another not 

 only along the surfaces of contact of their neural arches 

 but also at their ventrolateral edges of contact; the sixth 

 abdominal vertebra is interdigitated along the anterior 

 and posterior edges of its neural arch with the vertebrae 

 anterior and posterior to it; the first five abdominal 

 vertebrae are completely fused to one another. 



Caudal Skeleton. — The caudal complex consists of a 

 large rectangular plate, with a rounded expansion in the 

 middle region of its anterior edge representing the cen- 

 trum of the last caudal vertebra, and a deep cleft in the 

 middle of its posterior edge representing the division 

 between what in more generalized plectognaths would be 

 the second and third hypurals. However, the hypurals 

 are fully fused to the centrum and to themselves and no 

 real distinction can be made between them. Examina- 

 tion of young specimens shows that the anterior portion 

 of the caudal skeleton develops from a centrum which 

 has a complete neural arch and spine and haemal arch 

 and spine. The neural (epural) and haemal (parhypural) 

 apparatus in the 8.2 mm specimen already appear to be 

 continuous with the centrum, although a deep indenta- 

 tion in the dorsal edge of the plate partially separates the 

 epural from the fused hypurals. In all of the small 

 specimens a urostylar thickening can be seen projecting a 

 short distance posterodorsally from the posterior end of 

 the last vertebra, just in front of the anterodorsal edge of 

 the hypural region. This complex that represents the last 

 caudal vertebra corresponds to about the anterior one- 

 seventh of the adult caudal skeleton. Posterior to the 

 region of the last caudal vertebra in the 8.2 mm 

 specimen, and continuous with it, is the hypural plate, 

 representing the hypural elements fully fused already to 

 one another and to the last vertebra, except in the region 

 of the deep cleft in the posterior edge, the cleft being 

 much deeper in young specimens than in adults. In 

 adults all of these elements are simply more extensively 

 and indistinguishably fused into the single rectangular 

 plate already apparent at 8.2 mm. A small foramen is 

 present in the anteroventral region of the plate and this is 

 the last opening into the haemal canal, which forms a 

 short tube in the substance of the plate anterior to the 

 foramen and is continuous with the haemal canal of the 

 more anterior vertebrae. This foramen thus marks the 

 region of fusion of the lower hypural region with the 

 parhypural. The neural canal enters the anterior edge of 

 the plate just above the centrum and courses through the 

 plate to exit at a foramen on its posterodorsal edge. The 

 region of the plate above the neural canal represents the 

 fully fused epural. 



219 



