1028 



CLASS AMPHIBIA. 



956) consists of four portions — viz., a basal intercentrum (hypo- 

 centrum), a pair of pleurocentra, and a neural arch. In this rha- 

 chitomous type Professor Cope regards the pleurocentra as repre- 

 senting the centrum of the em- 

 bolomerous type, since they both 

 carry the arch ; and as he finds 

 that the functional centra in other 

 forms, like Chelydosaurus, appar- 

 ently correspond to the intercentra 

 of Archegosaurus, while the pleuro- 

 centra are small and apparently 

 about to disappear, it is argued 

 that in other Amphibia the real 

 centra are totally wanting, and 

 the vertebral bodies, which in the 

 caudal region have the chevrons 

 united to them, are really inter- 

 centra, to which the neural arches have been shifted. Professor 

 Cope regards the rhachitomous and embolomerous structures as char- 

 acters of at least family value ; but Dr Fritsch considers that the two 

 types occur in different regions of one and the same species, as we 



Fig. 956. — Diagram of a rhachitomous 

 vertebra ; from the front and left side. 1, 

 Pleurocentra ; 2, Intercentrum ; d, Neural 

 spine. (After Fritsch.) 



Fig. 957. — Parts of skull and vertebral column of Trimerorhachis ins ignis ; from the Permian 

 of North America, a, Basi- and exoccipitals ; b, c, Lateral and posterior view of angle of 

 mandible ; d, e, Portions of vertebral column depressed by pressure ; i, Intercentra ; /, Pleuro- 

 centra. (After Cope.) 



know to be the case in some of the Ganoid Fishes (supra, p. 959), 

 where we find in Eurycormus and Aspidorhynchus a rhachitomous 

 type of vertebrae in the cervical and dorsal regions, and an embol- 



