OF THE PELYCOSAURIAN GENUS DIMETRODON. 11 



The parietal : The parietal lias a broadened horizontal upper portion which unites 

 by strong suture with the frontal, postorbital and the parietal of the opposite side but 

 does not join the postfrontal. The pineal foramen lies in about the middle of this 

 horizontal portion and completely posterior to the orbits. The descending portion of 

 the bone curves sharply outward and downward and joins the quadrato-jugal as 

 described above. 



The squamosal : The squamosal lies largely on the posterior and inner (toward 

 the median line) side of the parietal. Its lower end is widened and overhangs the 

 distal end of the opisthotic exactly as in the Sphenodon but in larger degree. The 

 relations of the parietal and squamosal are rather peculiar ; the squamosal forms the 

 posterior side of the parietal arch and reaches almost to the median line of the skull 

 thus forming the major portion of the posterior aspect of the upper part of the skull, 

 in the Sphenodon the parietal forms the posterior portion of the skull in the median 

 and does not pass under the squamosal till about the middle of the parietal arch. 

 This gives the squamosal an appearance of greater prominence on the back of the 

 Felycosaurian skull but the bones have essentially the same relations in both 

 forms. 



The cranial region is formed by a single complex bone composed of the closel}' 

 coossified basi occipital, supraoccipital, exoccipital, opisthotic and petrosal ; in none 

 of the specimens are there well defined sutures separating these bones so that they 

 must have united early in life. Figures 2 and 3, PI. V show this region in specimen 

 1 where it was found disarticulated and complete ; the same region in the other speci- 

 mens has been somewhat crushed l)ut show enough to make it evident that they are 

 of the same character as specimen 1. The following description is taken from a pre- 

 vious paper discussing specimen 1. (Case, '99.) 



" The occipital region closely resembles that of Sphenodon. The condyle is 

 formed by the exoccipitals and basioccipital. The exoccipitals meet in the median 

 line above, excluding the supraoccipital from any part in the foramen magnum. 

 Laterally they join the expanded proximal ends of the opisthotics. The supraoccip- 

 ital is a triangular plate inclined forward as it ascends and joining by the base of the 

 triangle the parietals above. Laterally it joins the opisthotics and inferiorly the 

 exoccipitals. The opisthotics are expanded proximally, joining the supraoccipital and 

 exoccipitals. Distally they are elongated outwards, ])ackwards and downwards. The 

 lower edge of the proximal end is marked by a notch which, in union with similar 

 notches in the basioccipital and petrosal form the fenestra ovalis. The opisthotics 

 remained free during life or until advanced age. This feature is found only in turtles, 

 Ichtkyosaurs and the young Sphenodon. It has been noticed in young lizards before 



