48 



PERMO-CARBONIFF.ROUS VERTEBRATES FROM NEW MEXICO. 



the intercentral space, the latter with the arch at the extremity of a more or less 

 prominent diapophysis. And this is the primitive rib attachment in reptiles. In 

 the further evolution of the ribs these two surfaces, primitively continuous, become 

 separated by an emargination, leaving a distinct foramen between the articulated 

 rib and its vertebra. This modification of the articulation was, however, acquired 

 in different phyla independently, since we find the continuous articulation, the 

 "single-headed" rib, in such diverse reptiles as the "Microsauria," Diadectes, Labi- 

 dosaurus, Ophiacodon, Paleohatteria, and Sphenodon, while the emarginate form, or 

 "double-headed" rib, was acquired in such a generalized cotylosaur as Seymouria 

 (fig. 28 b). In all these reptiles, however, the ribs are distinctly double-headed in 

 the proper sense of the word, since they have both capitulum and tuberculum. Per- 

 haps some convenient single word to characterize each type, such as holocephalous 

 and dichocephalous , will be useful. 



Ophiacodon not only has holocephalous ribs, but also dilated cervical ribs, a 

 character (so far as known) hitherto peculiar to the Cotylosauria. Beyond the 



Fig. 28. — ^A, cervical ribs of Ophiacodon mtrus Marsh, X K- B, dorsal rib of Seymouria 

 baylorensis Broili, X i; C, right ilium of Ophiacodon mirus Marsh, inner side, X /^. 



seventh the ribs are all cylindrical and slender to their extremities, becoming quite 

 short in front of the sacrum. The first three cervical ribs, those articulating with 

 the atlas, the axis, and the third vertebra, are short, and but little dilated distally, 

 attached exclusively, it seems, to the diapophysis. 



Ventral ribs: The size and characters of the ventral ribs are best shown in the 

 photographic illustration (pi. i, fig. i). They formed a large sheet in the specimen 

 as found, covering the whole of the space back of the coracoids and between the 

 ends of the ribs, quite to the pelvis. They are very slender, and lie closely together, 

 each meeting its mate in the middle in an acute V. The length of no individual 

 rib can be determined, since they are all more or less broken into short, contiguous 

 pieces. It is possible that some of them, if not many, were continuous throughout 

 their whole lengths. 



Pectoral girdle and extremity: The pectoral girdle and both anterior extremities 

 lay quite in natural arrangement as found in the specimen, except that the right 

 scapula had been pushed downward and a Uttle forward, an inch or two from its 

 natural place ; and the left hand was partly concealed beneath the posterior dorsal 

 vertebras. The right arm and hand, closely articulated throughout, were directed 

 downward and forward ; the inner fingers were doubled under the outer, ulnar ones. 



The inter clavicle, preserved in place, has a broad, thin, flattened stem, some- 

 what convex below, and a moderately dilated anterior extremity. The stem 

 extends backward, as articulated, about an inch beyond the posterior angles of the 

 coracoids. In the present specimen (No. 650) it is somewhat flattened by pressure; 



