82 AMERICAN PERMIAN VERTEBRATES 



which has been restored complete; a half-dozen ilia, of which one 

 of the smallest I have figured; at least three pubes, and as many 

 ischia, none of which has been made complete; five femora, four 

 of about equal size, one of which I have figured, the fifth, though 

 no longer and resembling the others in shape, is a distinctly stouter 

 bone; six tibiae of nearly equal size, one of which is figured; four 

 fibulae of nearly equal size, one of which I figure; and numerous 

 hand and foot bones, many of which probably belong with this 

 genus. Besides these limb bones there are numerous vertebrae, 

 of which five or six are in one series, with the intercentral bones in 

 position, probably the ones referred to this genus by Marsh. I 

 figure an isolated one from the later collections. 



From the later collections, as already stated, I find no limb or 

 skull bones of these forms, though there are not a few vertebrae 

 with short, thin spines. Among these later collections there are 

 a number of mandibles and maxillae agreeing quite with the type 

 of Sphenacodon ferox, and a few limb bones which, though incom- 

 plete, seem to differ in their greater slenderness and more Dime- 

 trodon-like form. 



I refer these limb bones and vertebrae to this genus, rather 

 than to Sphenacodon, because they agree better with the polio- 

 saurid than the clepsydropid type, though their positive identity 

 must depend upon their future discovery in actual juxtaposition. 



The figures of the typical specimens of mandible, together 

 with Marsh's description, will render the skull of Ophiacodon 

 easily recognizable. The anterior teeth are long and recurved, and 

 somewhat flattened, not only in these specimens, but also in impres- 

 sions of perfect teeth yet preserved in the matrix. The posterior 

 teeth are almost cylindrical and scarcely at all compressed; they 

 are all shorter, and but slightly recurved. Several of the mandibles 

 are somewhat larger than in the type specimen. The mandible 

 may have had a length of from 150 to 200 millimeters. The 

 specimens indicate a relatively slender and small skull of the polio- 

 saurid type, too small, it would seem, to belong with some of the 

 limb bones, were it not there is similar disparity in size shown 

 between the mandibles and limb bones in the specimens of Thero- 

 pleura retroversa Cope figured by Case (Pelycosauria, Plate III) . 



