vertebrae), have their centra anteroposteriorly com- 

 pressed, the degree of which decreases posteriorly in the 

 series, and by the fact that the third to eighth abdominal 

 vertebrae have transverse processes laterally from the 

 centra. The posterolaterally directed transverse process 

 of the third abdominal vertebra arises from the upper 

 part of the centrum and the lower part of the neural arch. 

 The processes of the fourth and fifth abdominal 

 vertebrae are placed successively lower on the centra and 

 the processes of the sixth to eighth vertebrae are low on 

 the ventral edge of the centra. None of these abdominal 

 vertebrae, up to and including the eighth, have haemal 

 arches or spines. The ninth and arbitrarily designated 

 last abdominal vertebra has a short haemal arch and 

 spine differing from those in the caudal series only by its 

 smaller size. The neural spines of the third to seventh ab- 

 dominal vertebrae lie anterior to the first basal pterygio- 

 phore of the dorsal fin. The neural spines of the third, 

 fourth, and sometimes fifth abdominal vertebrae are 

 held distally in a narrow concavity along the ventral edge 

 of the ventral keel of the supraneural element or carina, 

 while the neural spines of the fifth to seventh vertebrae 

 simply articulate by fibrous tissue with the ventral edge 

 of the keel. The neural spines of the eighth and ninth ab- 

 dominal vertebrae articulate by fibrous tissue between 

 the basal pterygiophores of the anterior region of the dor- 

 sal fin. The haemal arch and spine of the ninth abdomi- 

 nal vertebra is symmetrically placed in the midline 

 below the centrum and there is no indication here or on 

 any of the more posterior vertebrae that the haemal 

 canal lies other than in the midline, in contrast to the 

 condition in the Ostraciidae. The neural spine and arch 

 of the seventh abdominal vertebra bears a low ridge 

 along its length, while a similar ridge is present on the 

 eighth and ninth abdominal vertebrae, but much more 

 prominently developed, ending ventrally on the anterior 

 end of the centrum as a flange. 



Caudal Vertebrae. — The caudal vertebrae numbered 

 nine in two specimens. All of the caudal vertebrae pos- 

 sess complete neural and haemal arches and spines. The 

 haemal spine of the first caudal vertebra is better de- 

 veloped than that of the last abdominal vertebra, but 

 still shorter than that of the second caudal vertebra, 

 which is the largest in the series. Posterior to the second 

 caudal vertebra the haemal spines decrease gradually in 

 size in the series to the seventh caudal vertebra. The 

 haemal spine of the eighth caudal vertebra is larger than 

 that preceding it and it is autogenous to the centrum. 

 The neural spines decrease in length gradually in the 

 series from the first to the seventh caudal vertebrae, 

 while the neural spine of the eighth caudal vertebra is 

 somewhat larger than that preceding it. The haemal 

 spines of the first to fourth caudal vertebrae support the 

 distal ends of the last four basal pterygiophores of the 

 anal fin, these being the only four which consistently lie 

 in the midline of the body, rather than being divergent 

 from it as are the more anterior pterygiophores. The 

 neural and haemal canals lie in the midline of the body 

 and are not divergent from it. Each of the caudal verte- 



brae, as with the abdominal vertebrae, possesses a neural 

 foramen low on the neural arch region. 



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 be- 

 tween what in more generalized plectognaths such as 

 triacanthodids 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 be- 

 tween them. The anteroventral region of the rectangular 

 plate represents the parhypural which is fully fused to 

 the centrum and hypurals. That this is the region of the 

 parhypural is indicated by the haemal canal penetrating 

 the anterior region of the rectangular plate just below the 

 centrum and exiting at a foramen a short distance pos- 

 terior to where it first enters the plate. This foramen 

 represents the region of fusion between the parhypural 

 and the hypurals. The anterodorsal region of the rec- 

 tangular plate represents the completely fused epural, 

 for the neural canal penetrates the plate, again just 

 above the centrum, and courses through it to exit about 

 midway along the dorsal edge of the plate. 



Caudal fin rays. — Eleven fin rays are present in 

 both specimens and generally throughout the Aracani- 

 dae; the uppermost ray and the lowermost ray un- 

 branched, the intervening nine rays branched; all rays 

 with cross-striations. The fin is vaguely divided into an 

 upper and lower lobe, the upper lobe containing five rays 

 articulated to the fused hypural plate above the indenta- 

 tion in its posterior edge and the lower six rays articu- 

 lated to the lower half of the plate. The bifid bases of the 

 rays do not overlap the hypural plate, to which they are 

 articulated through fibrous tissue. The branched fin rays 

 become increasingly branched toward the center of the 

 fin, where the rays are branched in up to incomplete 

 quadruple dichotomies. 



DORSAL AND ANAL FINS. 



Dorsal Fin. 



Fin rays and pterygiophores. —Eleven fin rays are 

 present in both study specimens; the first ray un- 

 branched, the other rays branched in single to triple 

 dichotomies. Distal pterygiophores are either absent or 

 unossified. The bifid bases of the fin rays are supported 

 through fibrous tissue by 9 or 10 basal pterygiophores. 

 The specimen with 9 basal pterygiophores (the il- 

 lustrated specimen) has the last pterygiophore a com- 

 pound element obviously representing the fusion product 

 of 2 pterygiophores, for in the other study specimen there 

 are 2 separate basal pterygiophores in this region for a 

 total of 10. With the exception of the first and last 

 pterygiophores the basal pterygiophores are more or less 

 slender rods, cartilage filled at both ends, and of slightly 



