84 



REVISION OF THE PELYCOSAURIA. 



The basisphenoid is quite similar to that of Dimetrodon^ but the anterior pro- 

 cess, the parasphenoidal portion, is much longer and heavier and extends well forward 

 in a straight line instead of being turned upward as a thin vertical plate. This is 

 one of the most primitive characters of the skull. 



The pterygoids of both sides are preserved in part. The posterior half is present 

 on both sides. There was the same division of the bone into an anterior and pos- 

 terior and an external portion as in Dimetrodon. The anterior portion is largely 



missing, but enough remains to show that it was flat 

 and covered with small teeth. The posterior portion 

 is nearly vertical, but inclined somewhat inward, and 

 the posterior end overlapped and articulated with the 

 inner side of the inner condyle of the quadrate. The 

 external portion stood out at right angles to the other 

 part of the bone, but was far from vertical, slanting 

 backward at a large angle. The outer face of the ex- 

 ternal process abutting against the lower jaw is rugose, 

 but very thin; there are fifteen small teeth arranged 

 in sockets on the lower side. The development 

 of the external process of the pterj'goid from 

 this slanting condition with a thin external face 

 to the perfectly vertical position with the outer 

 face broad and rugose is one of the clearest 

 lines of development in the suborder. 



There is no trace of the ectopterygoid. 

 The epipterygoid stands in the usual posi- 

 tion on the upper edge of the pterygoid. It is 

 thin with an elongate base parallel to and rest- 

 ing on the upper edge of the pter\-goid. 

 The lower jaws of both sides are represented by fragments, but these can not be 

 fitted together so as to give an accurate estimate of the length. The jaw was much 

 more slender than in Dimetrodon and of less width anteriorly, but was seemingly 

 quite high behind. The articular region was attached to the inner side instead of 

 being terminal in much the same way as in that genus, but the details can not be 

 made out. The anterior end was very slim, only 19 mm. high, the two sides united 

 in a long symphysis. The teeth are smaller than in the upper jaw and more numer- 

 ous. The anterior end of the jaw is suddenly swollen and accommodates the sockets 

 of enlarged teeth. The first tooth is small, followed by three much larger ones, 

 after which the teeth are abruptly smaller. There are nineteen in the first third of 

 the jaw; the total number can not be made out. 



I'ertebral column (plate 13): This consists of forty vertebrae which are in 

 connected series, except for a break just anterior to the two sacrals. The column as 

 preserved begins with the axis and ends with the ninth caudal ; the vertebrae are held 

 by a hard flinty scale of matrix and most are in the normal position ; but two or 

 three are turned through 90°. As indicated in the systematic revision the vertebral 

 column differs from that of the Clepsydropidcr in two points, the low neural spines 

 and the nearly uniform length of the vertebrte in all parts of the column. 



Fig. 22. — Stapes of Thezopleura relroversa. X /4. 



After Cope. a. Proximal end of same. 

 Fig. 23. — Lateral view of the basisphenoid of Thero- 



pleura relroversa showing the large parasphenoid 



rostrum. X 1. 



