234 Prof. T, H. Huxley. On [Mar. 31, 



Tig. 2. 

 N 



Caudal vertebra of Chelydra. Letters as in fig. 1. 



very much stronger and the pentagonal platform into which the upper 

 surface of the neural arch expands, in place of a neural spine, is as 

 long as the vertebra instead of being only about half as long. The 

 stout pre-zygapophysis of the right side is broken off, leaving only the 

 base visible in the fossil. 



Two other caudal vertebrae, having the same structural features, 

 occur among the detached remains ; and belong, like the first, to the 

 second fourth of the tail. Another tolerably complete vertebra, with 

 a considerably longer centrum, corresponds very closely with a caudal 

 vertebra of Gypochelys from the third fourth of the tail. In this, as 

 in one of the foregoing vertebrae, the chevron bones are ankylosed 

 with the centrum. I conceive, then, that there can be no doubt that 

 the pelvic bones and these caudal vertebrae belonged to a Chelydroid 

 Chelonian, of about the size of the largest " Snapping turtles" which 

 are met with in North America at the present day. 



question, and has pointed out the exceptional nature of their structure among the 

 Chelonia. Since the above paragraph was written, Dr. Grunther has kindly enabled 

 me to examine a spirit specimen and a skeleton of Platy sternum. The caudal 

 vertebrae resemble those of Chelydra, except that the last nine are procoelous, while 

 that between these and the more anterior opisthocoelous vertebrae is nearly flat at 

 the ends. In this, as in other respects, Platysternum presents characters inter- 

 mediate between Chelydra and the ordinary Emydce. Professor Cope (' Vertebrata 

 of the Tertiary Formations of the West,' 1883, p. Ill) ascribes opisthocoelous 

 caudal vertebrae to the Baenidce, but no figures or descriptions of such vertebrae are 

 given. Of the opisthocoelous Chelonian vertebrae figured in Plate XXIV of the 

 ' Report of Extinct Vertebrata obtained in New Mexico ' (1877) it is expressly stated 

 that their "correct reference cannot now be made " (p. 43). 



