44 PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



probability it is. It forms a sort of bridge between the atlas and the occiput, above 

 the spinal canal, with an obtuse protuberance or rudimentary spine above on 

 each side, like that on each atlantal arch. 



Atlas (fig. 24) : The atlas is preserved perfectly on the left side. Each neuro- 

 centrum is slender, curving back to articulate, by a well-formed zygapophysial 

 facet, with the axis at the base of the spine in front. On the front of each arch 

 above there is a zygapophysial facet, slightly constricted from the arch itself, for 

 articulation with the postzygapophysis of the proatlas. Anteriorly a process 

 descends downward and backward, articulating at its extremity with the atlantal 

 rib, which lies closely in position in the specimen. The neurocentrum, as a whole, 

 differs only a little from that figured by Williston in his American Permian Verte- 

 brates, plate XXXIV, figs. 10, 11, from the Baldwin quarry, the chief difference 

 consisting in the more slender process which is represented by a mere protuberance 

 in the atlas of Ophiacodon. In much probability the figured neurocentrum belongs 

 with Sphenacodon. The arch is supported chiefly by the odontoid, resting only 

 slightly upon the atlantal intercentrum below. The intercentrum is of somewhat 

 larger size than the intercentrum between the odontoid and the axis. 



The atlas, it is seen, is of a primitive type, almost identical in structure with 

 such vertebrae as are known in the tails of certain temnospondyls, with the omission 

 of the chevron. In fact, the union of the neurocentra with each other and with 

 the intercentrum would result in an almost typical embolomerous vertebra. 



This condition of the atlas in these primitive reptiles suggests certain conclu- 

 sions. The reptilian atlas could not possibly have been evolved from the atlas of 

 any known amphibian. In the temnospondyls the atlas is composed exclusively 

 of the paired neurocentra fused with the intercentrum; the pleurocentra have, 

 apparently, disappeared entirely. The proatlas also is quite unknown in any 

 amphibian. The proatlas is generally regarded as the remnant of a preatlantal 

 vertebra which has disappeared in the reptiles, and the presence of zygapophysial 

 articulations in these early reptiles would substantiate that theory. Gadow, 

 however (Evolution of Vertebral Column, etc.), insists that the proatlas is merely 

 the spine of the atlas. It has been known for some time that the amphibian occi- 

 put, whether of living or ancient forms, does not correspond to the reptilian; and 

 we also know that the atlas in the two classes is not homologous. If, then, the cor- 

 roborative proof furnished by these reptiles supports the theory that the reptilian 

 atlas is in reality at least the second, perhaps the third vertebra of the amphibians, 

 it would seem probable that the amniote atlas was derived from a true holospondy- 

 lous vertebra, but one in which the primitively large intercentrum was persistent ; 

 perhaps such a vertebra as is found in Seymouria. 



Among modem reptiles a proatlas is known in the Crocodilia and Sphenodon; 

 also in certain dinosaurs and the Therapsida. In Procolophon, as Broom informs 

 us, the atlas and proatlas are almost identical with those of the Pelycosauria, except 

 that the intercentra in front and behind the axis are parial, a condition also found 

 in some modem turtles. In the Dinocephalia, according to the same authority, 

 the atlas and proatlas seem to be typically pelycosaurian in structure. Indeed, 

 it may be assumed that this condition is that of all primitive reptiles. Brown 

 (Osteology of Champsosaurus) describes the arch of the atlas in the very archaic 

 genus Champsosaurus as articulating directly with the exoccipital by an anterior 

 process. This condition would be remarkable, and one wonders whether there 

 may not have been an intervening proatlas which has not been recognized. 



The axis (fig. 24) differs from that of Dimetrodon especially in the much greater 

 antero-posterior expansion of its spine, which is even greater than in other known 



