98 REVISION OF THE PELYCOSAURIA. 



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 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 

 paroccipital exactly as in Splienodon, but in larger degree. The relations of the parie- 

 tal 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 Sphenodon 

 the parietal forms the posterior part of the skull in the median line and does not pass 

 under the squamosal till about the middle of the parietal arch. This gives the squamo- 

 sal an appearance of greater prominence on the back of the Pelycosaurian skull, but 

 the bones have essentially the same relations in both forms. 



The cranial region is fonned by a single complex bone composed of the closely 

 coossified basioccipital, supraoccipital, exoccipital, paroccipital, and petrosal ; in none of 

 these specimens are there well-defined sutures separating these bones, so that they must 

 have united early in life. Plate 11, figs. 7 and 8, shows this region in No. i University 

 of Chicago, where it was found disarticulated and complete; the same region in the 

 other specimens has been somewhat crushed, but shows enough to make it evident that 

 they are of the same character as specimen i. The following description is taken 

 from a previous paper discussing specimen i (Case 22) : 



"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 supraocci- 

 pital 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, backwards, and downwards. 

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

 lar 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, 

 Ichthyosaurs, and the young Sphetwdoti. It has been noticed in young lizards before 

 leaving the ^<g%.'' The basioccipital forms the lower portion of the condyle and lies 

 between the exoccipitals and opisthotics. The lower surface is trough-like for its 

 posterior half and supported a posterior extension of the basisphenoid. Laterally a 

 slight notch forms the inner wall of the fenestra ovalis. Anterior to the horizontal 

 trough-like portion the inferior surface rises sharply ; the angle thus formed is marked 

 by a large foramen of unknown function, perhaps the hypoph>'sis passes into the 

 interior of the basioccipital, plate 11, fig. 8. The petrosals join the opisthotics, exoc- 

 cipitals, and the basioccipital, but the sutures are not distinguishable. The lower 

 part of the anterior edges was continued forward as long processes, the anterior-inferior 

 processes of Siebenrock.f These are partially destroyed in the specimen. A deep 

 notch in the anterior edge of the petrosals just above the origin of these processes, 



* Siebenrock, F. : Das Skelet der Lacerta Simonyi Steind, und der Lacertiden familie uberhaupt ; 

 Sitzunberichten der kaiserl. Akademie der Wissenschaften in Wien. Mathm. Naturwiss. Classe., cm, Abth. 

 I, April, 1894. 



tSiebenrock, F.: Zur Osteologie des Hatteria-Kopfes, ibid., Bd. cii, Abth. i, June, 1893. 



