caudal vertebrae are the largest of those of the caudal 

 vertebrae. In large specimens (over 200 mm) these two 

 haemal spines become enlarged and swollen, taking on 

 the hyperostotic consistency that is also characteristic of 

 the ethmoid in large specimens. The haemal structures 

 of the last two vertebrae are autogenous. 



The neural spines of the first five caudal vertebrae are 

 slender rods which support most of the basal pterygio- 

 phores of the dorsal fin. They arise from the posterodor- 

 sal surface of the neural arches at the region of the neural 

 postzygapophyses. The neural spine of the first caudal 

 vertebra is placed between the third and fourth basal 

 pterygiophores and that of the fourth caudal vertebra 

 behind the ninth and last basal pterygiophore. The 

 neural spine of the fifth caudal vertebra is larger than 

 those of the caudal series anterior to it, but it is rela- 

 tively small in comparison to the neural spine of the sixth 

 caudal vertebra. Posterior to the sixth caudal vertebra 

 the neural spines become progressively smaller. It is of 

 interest to note that whereas the haemal spines of the 

 sixth and seventh caudal vertebrae are swollen and 

 spongy in the two largest study specimens, the neural 

 spines of these same vertebrae are normal thin hard 

 plates. 



Caudal Skeleton. — The supporting structure of the 

 caudal fin appears to be a single large block of bone, 

 square in its lateral outline, but it is a compound struc- 

 ture formed of four separate bony elements firmly inter- 

 digitated with one another. The ventral portion of the 

 caudal skeleton is formed by the parhypural. A slight 

 concavity is present in about the middle of its dorsal sur- 

 face just below a similar concavity on the ventral surface 

 of the bony element that lies above it, which is the fused 

 centrum-lower hypurals. The parhypural and the fused 

 centrum-lower hypurals are extensively interdigitated 

 with each other, except in the region where the concavi- 

 ties occur in their apposed surfaces so that a foramen is 

 enclosed between them. This foramen is the last opening 

 into the haemal canal, the latter being connected to the 

 canal in the preceding vertebrae by a small longitudinal 

 groove along the articular surface between the 

 parhypural and the centrum-lower hypural element 

 anterior to the foramen. Anteriorly the parhypural is 

 laterally expanded to form haemal prezygapophyses. 

 The epural is much like the parhypural in shape, but it is 

 shorter and only forms the dorsal margin of the anterior 

 two-thirds of the caudal skeleton. Along most of its ven- 

 tral surface it firmly interdigitates with the centrum but 

 posteriorly it interdigitates with the autogenous upper 

 hypural plate. Concavities exist on the ventral and dorsal 

 surfaces, respectively, of the epural and centrum so that 

 a foramen is enclosed by the otherwise apposed surfaces 

 of the two bones. A deep longitudinal groove is present 

 along the ventral surface of the epural and the dorsal sur- 

 face of the centrum which permits the neural canal to be 

 present in the midline of the otherwise interdigitated sur- 

 faces of these two elements. The neural canal opens 

 anteriorly into the neural canal of the preceding verte- 

 brae, while posteriorly it opens to the exterior between 



the posterior end of the epural and the dorsal surface of 

 the autogenous upper hypural plate. The anterior end of 

 the epural is expanded laterally to form the neural 

 prezygapophyses. The fused centrum-lower hypurals 

 form the largest bone of the caudal skeleton, its anterior 

 portion corresponding to the last vertebral centrum and 

 abbreviated urostyle, and its posterior portion corres- 

 ponding to what in generalized plectognaths would be 

 the first and second hypurals. The anterior portion inter- 

 digitates dorsally with the epural, posterodorsally with 

 the upper hypural plate and ventrally with the 

 parhypural. The posterodorsal region of the anterior por- 

 tion, which represents the urostyle, is extensively inter- 

 digitated with the anteroventral edge of the upper 

 hypural plate. The posteroventrally expanded portion 

 representing the lower hypurals articulates by interdigi- 

 tation dorsally with the upper hypural plate and ventral- 

 ly with the parhypural. There is a short concavity, or 

 blindly ending tube, proceeding posteriorly and slightly 

 dorsally into the lower hypural region from the area of 

 the haemal foramen between the centrum and parhy- 

 pural. This tube probably encloses the end of the haemal 

 canal. The lower hypural region supports the lower six 

 caudal fin rays, with the support of the lowermost ray 

 shared with the parhypural. The autogenous upper 

 hypural plate forms the posterodorsal region of the 

 caudal skeleton and represents what in generalized plec- 

 tognaths would be the third to fifth (or, in Triodon, third 

 and fourth) hypurals. It interdigitates strongly with the 

 centrum-lower hypural plate and with the posterior end 

 of the epural. The upper hypural plate supports the up- 

 per five caudal fin rays. 



Caudal fin rays. — Eleven in number, the upper- 

 most ray and lowermost two rays unbranched, the inter- 

 vening rays becoming increasingly branched toward the 

 middle rays, which are branched in at least triple dicho- 

 tomies. The upper five rays articulate by fibrous tissue at 

 their bifid bases to the upper hypural plate and the lower 

 six rays mainly to the lower hypural region, as described 

 above. 



DORSAL AND ANAL FINS. 



Fin rays and pterygiophores.— Fi/tecn fin rays pres- 

 ent in most specimens, the first two rays and the last one 

 or two rays unbranched, the others branched in single or 

 double dichotomies. The first ray is small and rudimen- 

 tary, barely protruding to the surface, formed of two 

 separate halves and without cross-striations, while all 

 the other rays are cross-striated. Distal pterygiophores 

 are either absent or unossified. The fin rays are sup- 

 ported below by nine basal pterygiophores which are 

 strongly interdigitated with one another dorsally. The in- 

 temeural processes of these pterygiophores are large 

 anteriorly in the series but decrease in size posteriorly. 

 The first and second basal pterygiophores are placed well 



