AMERICAN LABYRINTHODONTIDM 605 



decided that pleurocentra were not present in the axis. There is a 

 possibility that they have been lost in the specimens examined. 

 Cope's figure also shows that the pleurocentra of the third cervical 

 are very small. He figures no caudals except five proximal ones, in 

 which the pleurocentra are greatly reduced and the intercentrum 

 bears the arch, and five at the end of the tail, in which the intercentra 

 are large and the pleurocentra very small. 



The absence of the pleurocentra in the atlas and axis, and their 

 reduction in the caudals, force the following conclusions: Eryops 

 is not near the direct ancestry of the Amniota, because in all primitive 

 Amniota the pleurocentra are large in the atlas and axis. On the 

 contrary, the tendency is toward the development of true stereospondy- 

 lous vertebra?, as in the Labyrinthodontidae. The atlas and axis 

 are composed of the same elements as in that form, and the caudals 

 are approaching that type by the reduction of the pleurocentra. 

 This conclusion was reached independently before the writer had 

 read Jaekel's paper on Archegosaurus, 1 in which he reaches a like 

 conclusion concerning the vertebrae of that genus, because of the great 

 reduction of the pleurocentra in the caudals. 



The problem of the homologies of the elements in the temno- 

 spondylous stegocephalian vertebrae has been much confused by 

 misapprehension on the part of some authors, and by a lack of full 

 information concerning the structure of this part of the skeleton of 

 some of the forms that have been discussed most. The status of 

 opinion to date is about as follows: 



All writers are agreed that the neurapophyses are homologous 

 with the neurapophyses of the Amniota. Gaudry 2 and Fritsch 3 

 believed that the intercentrum (hypocentrum) is the true centrum 

 (pleurocentra) of the Amniota, while Cope, Baur, Albrecht, Dollo, 

 and others maintain that it is homologous with the intercentrum of 

 the Amniota. The evidence is overwhelmingly in favor of the latter 

 view, and it is the one now generally accepted. But the homologies 

 of the elements called pleurocentra and hypocentra pleuralia are 



1 Zeitschrift der Deutschen geologischen Gesellschaft, 1896. 



2 "Les enchainements du monde animal," Fossiles primaires (Paris, 1883). 



3 Fauna der Gaskohle und der Kalksteine der Permjormation Bolimens, Vol. II 

 (1889). 



