PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 65 



Pelvis (fig. 40 c) : The pelvis is represented by a portion of the left half, show- 

 ing the cotylus and the outer part of the pubis. A much better specimen of the 

 Yale collections is shown in fig. 43 d. 



The cotylus is rather narrow and deep, with prominent edges on all of the bones; 

 in this it differs from Dimetrodon, in which the edge of the cotylus on the pubis is 

 not elevated. This permitted the femur of Dimetrodon to be extended much more 

 directly forward than in Sphenacodon, in which it must have stood almost directly 

 out from the side of the pelvis. The ilium is too incomplete for any detailed descrip- 

 tion ; the crest was broad, differing notably from that of Theropleura, which is narrov/, 

 resembling the ilium of Varanosaurus. That it was firmly attached to the sacrum 

 is shown by the prominent rugosities on the inner side. In Dimetrodon there is a 

 narrow process on the proximal edge of the ilium, which forms a buttress against 

 which the end of the femur fitted; in Sphenacodon the whole edge is elevated, 

 though there is a narrow portion on the posterior part which carries the face for 

 the femur. The outer edge of the pubis is thickened ; it runs directly forward from 

 the cotylus and is slightly deflected at the distal end. The opening of the pelvis 

 was rounder than in Dimetrodon, in which it was distinctly angular at the lower 

 part. The pubic foramen perforates the bone at a point just below and anterior 

 to the cotylus. 



The vertebral column is represented by an atlantal intercentrum, an axis, 

 several vertebrae associated with the skull, and ten others closely associated to- 

 gether. The intercentrum is broad below and of good size. Its anterior face is 

 deeply concave, both vertically and horizontally, indicating a well-rounded con- 

 dyle. Its posterior face is smaller and more shallow; the posterior edge of the 

 lower surface has a shallow notch in the middle. The presence of atlantal ribs is 

 indicated by small facets on the upper part of the posterior margin. The axis 

 resembles that of Dimetrodon much more closely than that of Ophiacodon or Thero- 

 pleura. In the latter the spine is short and the distal end is fully as long, antero- 

 posteriorly, as the rest of the spine ; the anterior and posterior margins are nearly 

 vertical, with no projection of the lower part of the anterior end forward over the 

 atlas. The axis of Sphenacodon (fig. 40 d, e) has an elevated neural spine, the 

 lower part of which extends forward, terminating in a slight rugosity, beyond which 

 the border slopes upward and backward slightly concave in outline to the rather 

 wide and triangular apex. 



The posterior border of the spine is nearly vertical and for its lower two- 

 thirds deeply excavated by a narrow groove, which increases in depth downward 

 until it reaches its maximum at the level of the posterior zygapophyses. The 

 edges of this groove are sharp, and there is a small but prominent knob or process 

 just over each posterior zygapophysis. We have never seen such a process on the 

 axis of Dimetrodon or other pelycosaur. The anterior zygapophyses are small, 

 but well formed; they look almost directly upward and but slightly outward. 

 The posterior zygapophyses are relatively large for the size of the axis. The 

 anterior face of the centrum is elongate and heart-shaped, due to the development of 

 a sharp keel on the lower surface. On the upper edge of the articular face there are 

 two large, elliptical, concave faces, centantra, looking slightly inward and down- 

 ward; they are relatively much larger than any observed in Dimetrodon. The 

 posterior face of the centrum is circular, with no faces on the upper margin corre- 

 sponding to those of the anterior face. 



Centantra and centrosphenes do not seem to occur on the post-axial vertebrae. 

 The sides of the centra are deeply concave and terminate below in a sharp keel, 

 which has a very sharp line on its lower surface. The keel and concavities of the 



