PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



67 



arch, with the broad upper surface on a level with the anterior zygapophyses. It 

 is evident that these vertebree closely resemble those of Dimetrodon, especially the 

 form described as D. giganhomogenes by Case, but the faces of the centra are rela- 

 tively more elongate and the longer transverse processes do not dip so far down- 

 ward, as is common in all species of Dimetrodon. As compared with Theropleura, 

 which Sphenacodon resembles in some respects, the centra are much narrower 

 below and the sides are concave, the faces of the anterior dorsal vertebrae are 

 more elongate, and the spines are higher and narrower. 



It is apparent that Sphenacodon was closely related to Dimetrodon. The skull 

 is, so far as can be determined from known material, almost identical; the few 

 resemblances to Theropleura which have been pointed out are all of a primitive 

 character. We regard Sphenacodon as a less specialized member of the group of 

 dimetrodonts, which had not yet acquired those habits, whatever they were, which 

 led to the excessive development of the spines and the more elongate and slender 

 limbs. The position of the femur, directed nearly straight outward, indicates a 

 more sprawling habit of body. Perhaps the animal had not yet so thoroughly 

 adapted itself to the dry-land habitat which seems to be characteristic of the 

 specialized, long-spined forms. 



Fig. 42. Sphenacodon ferox Marsh, No. 818, Yale University, X Yi- A, basioccipital, intercentra, and 

 odontoid seen from the side; B, left side of an axis; C, anterior view of a posterior cervical vertebra; 

 D, lateral view of a dorsal vertebra; E, anterior view of a posterior cervical vertebra. 



Note by Williston. The foregoing material collected by us in New Mexico is of great 

 interest, but leaves in doubt several important problems as to the relationships and struc- 

 ture of this genus; this doubt is partly, not entiiely, dissipated by the better collections of 

 the Yale Museum, made by Baldwin so long ago. Williston had the privilege of study- 

 ing this material, though not as fully as he desired, the past summer. Several specimens, 

 which must be referred to the genus Sphenacodon, have been thoroughly prepared, and 

 settle beyond dispute the general characters of the genus, though it is by no means 

 certain that they all belong to the same species as that originally named by Marsh from 

 material from the Baldwin quarry. The most important and best preserved of these speci- 

 mens is No. S18 in the Yale collections, coming from the "Rito Puerco." It was embedded 

 in a red sandstone, and the bones are somewhat crushed, and many fragments are missing. 

 It includes the two innominata, scapula, humerus, part of skull, axis, about twenty verte- 



