266 S. W. WILLISTON NEW GENERA OF PERMIAN VERTEBRATES 



diminished, leaving a vestige only in the intercentrum of the modern 

 8phenodon and other reptiles, and possibly certain mammals. On the 

 other hand, Cope, at least, believed that the pleurocentra disappeared in 

 the holospondylons and modern amphibians, the hypocentrnm persisting 

 as the centrum. 



The other view, that of various authors, notably Gaudry, Goette, 

 Gadow, Jaekel, and Broili, is that the pleurocentra and hypocentrnm fuse 

 to form the centrum of all the higher vertebrates. 



Unfortunately, both of these views must remain as hypotheses until, it 

 is hoped, some fortunate discovery of intermediate forms may confirm or 

 reject one or the other or both. In favor of the second view, which has 

 been well presented by Broili,^ it is seen (plate 12, figure 5) that the 

 pleurocentra articulate with each other in the middle, inclosing below 

 and between them and the h3^pocentrum a notochordal perforation, quite 

 analogous, perhaps homologous, with the central perforation in all primi- 

 tive holospondylons vertebrae. With a close fusion of the four elements 

 of the vertebrae and obliteration of the sutures a rhachitomous vertebra 

 would be practically indistinguishable from a notochordal vertebra of the 

 early reptiles. Moreover, the presence of distinct elements in the primi- 

 tive cartilaginous vertebrae of the chick, in addition to the arch, assumed 

 to be pleurocentra and hypocentra, would suggest their persistence closely 

 fused in the ossified centrum. 



This explanation would seem to be the most reasonable of any, 

 especially if we had to deal with the modern amniota alone. Unfortu- 

 nately, there are objections, ones recognized by Cope and Baur, which 

 complicate matters. In the first place, this theory would necessitate the 

 recognition of a morphological distinction between the intercentra as 

 originally so called by Cope, and as found in the living Sphenodon, and 

 the primitive hypocentra. It is a well known fact that intercentra are 

 far more prevalent among the old forms than the modem ones; that 

 practically all reptiles with notochordal perforations possessed them, 

 while only vestiges are found among niodern reptiles save Sphenodon, 

 leaving for the present the chevrons out of account. In the modern 

 crocodilia alone has every vestige of them disappeared back of the atlas, 

 while the lizards possess them throughout the neck, and turtles have at 

 least one or two back of the atlas. If they are morphologically distinct 

 elements, what explanation can be given for their origin and wide preva- 

 lence among the early reptiles and their gradual disappearance in the 

 modern forms? Thev are known onlv in the amniota. Neither of the 



•Monatschr. d. Deutschen Geologisb. GeseUsch., ix, 190S, p. 236, 



