606 E. B. BRANSON 



still in dispute. All are agreed that the posterior elements in the 

 vertebrae of Eryops are homologous with the centrum of the Amniota, 

 but Cope, Baur, and others believe that they are also homologous 

 with the pleurocentra of Archegosaurus, Actinodon, and other tem- 

 nospondylous stegocephalians ; while Gadow holds that they are the 

 hypocentra pleuralia of Archegosaurus and Chelydosaurus. 1 



These elements of Eryops are ventralia, not dorsalia, consequently not 

 pleurocentra. They are interventralia enlarged and extending upwards. Con- 

 sequently they are homologous with Fritsch's hypocentra pleuralia of Chelydo- 

 saurus, of the tail of Archegosaurus and homologous with the centra of the Amni- 

 ota. 



Cope, Baur, Albrecht, Dollo, and others believe that the hypocentrum 

 pleurale is united either to the intercentrum 2 in front, or to the pleuro- 

 centrum 3 above in Eryops; or, as Baur puts it, "Das Hypocentrum 

 pleurale tragt zur Vervollstandigung des Wirbelkorpers bei." 4 



Jaekel has clearly shown 5 that in the caudal region of Archego- 

 saurus the pleurocentra elongate and each separates into two elements, 

 one above and one below, the lower being the hypocentrum pleurale 

 (interventralia of Gadow). Probably no separation of this kind takes 

 place in Eryops, since in the anterior caudals the pleurocentra are 

 reduced and pushed high up by the close approach of the intercentra. 

 A comparison of Figs. 14 and 15, and 16 and 17, shows how closely 

 the vertebras of Eryops resemble those of Archegosaurus. In the 

 thoracic region there can be no doubt about the homology of the 

 pleurocentra of the two forms. In the caudal region the evolution 

 has progressed along different lines. In Eryops the hypocentra 

 pleuralia do not separate from the pleurocentra, but both are reduced 

 together; but the difference is not sufficient to raise a doubt about 

 the homologies of the parts. 



Dr. Hans Gadow calls Eryops a reptile on account of the structure 

 of its vertebrae, but- his conception of the vertebrae of this animal 

 was wrong in several particulars. He based his conclusions on the 



1 Philosophical Transactions of the Royal Society of London, Vol. CLXXXVII 

 (1896), p. 22. 



2 Proceedings of the American Philosophical Society, Vol. XVI, p. 245. 



3 American Naturalist, 1886, pp. 76, 77. 



4 Biologisches Centralblatt, Vol. VI, p. 333. 



5 Zeitschrift der Deutschen geologischen Gesellschaft, 1896. 



