62 PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



Second, an imperfect skull, including a nearly complete lower right ramus, 

 lacking only the articular region ; the anterior part of the left ramus, almost identi- 

 cal with the fragmentary holotype ; the imperfect right maxilla with part of the 

 nasals ; a less perfect left maxilla ; the external process of both pterygoids ; both post- 

 orbitals ; the lower border of the left orbit ; a complete left quadrate ; and a fragment 

 of the right quadrate. With this skull were closely associated many other bones, 

 including two complete vertebrae, three less complete, two clavicles, and a femur. 

 From the Miller quarry. 



Third, a poorly preserved skeleton, of which two dorsal vertebras, part of the 

 right maxilla, and a fragment of the scapula have been cleaned. This specimen 

 was found isolated, embedded in a soft gray sandstone. 



Other bones regarded by us as belonging to this genus are two scapula-cora- 

 coids, an imperfect pelvis, an axis, atlantal intercentrum, and three imperfect 

 maxillae, all from the Miller quarry. 



Skull: It is not surprising that Case, in his first description of this genus, 

 referred it to Dimetrodon, since, with all the fragments of the skull before us, 

 representing a goodly portion of its structure, we are unable to discover a single 

 character that would separate the form generically from that genus. It is prac- 

 tically only in the vertebrae that a distinction is possible. Williston has pointed 

 out that the number of teeth in Sphenacodon is less than in Dimetrodon; he counted 

 twenty-four in the holotype mandible; in the right mandible before us there are 

 twenty-two, and, allowing for the presence of one anterior to the enlarged teeth 

 and one at the posterior end, there would be exactly twenty-four. In the left 

 mandible, making allowance for four which had been broken away from the upper 

 margin, and for one in front of the enlarged tooth, and one at the posterior end, 

 there are also twenty-four. The maxillae are all imperfect, but they certainly had 

 less than twenty teeth each. Williston counted sixteen in a maxilla associated with 

 the holotype mandible. In the specimens before us the largest number is fourteen, 

 with a possible maximum of three more, making seventeen. Case, from a study 

 of numerous specimens of Dimetrodon, concluded that the number could not have 

 been far from twenty-eight in the mandible and twenty in the maxilla. None of 

 the teeth, as mentioned by Williston, are crenulate. In one specimen the bases of 

 the large teeth are quadrangular in outline, as described by Case for Theropleura. 

 The upper margin of the diastemal notch, anterior to the enlarged maxillary tooth, 

 is nearly on a line with the alveolar border of the maxilla ; that is, it does not ascend 

 as in the more specialized members of Dimetrodon (e.g., D. gigas). In other char- 

 acters, so far as the material we have studied goes, the skull of Sphenacodon can 

 not be distinguished from that of Dimetrodon, and certainly none of these characters 

 can be considered of generic value. 



The premaxillcB in two specimens show but two large teeth in each, the anterior 

 one much larger than the second. Williston has figured a specimen from the Yale 

 collection with a third, posterior tooth, much smaller than the others. 



The maxilla (fig. 38) has two enlarged teeth, which seemingly alternated in func- 

 tion, as in one of them the anterior tooth is the larger and the following one .shows 

 evidence of incomplete development. In another specimen the second tooth is the 

 larger. This seems to bear out the conclusion expressed by Case that the large teeth 

 in Dimetrodon alternated in function — that a single one performed the main work 

 while the other was maturing, although in many specimens the two teeth are of 

 nearly equal size. In the skull under description there are no teeth in the notch 

 anterior to the enlarged tooth, but in other specimens in the collection, and also in a 

 specimen in the Yale collection, as figured by Williston, there are two small teeth. 



