i86 



S. W. WILLISTON. 



there is perhaps good reason for the belief that the original 

 lateral fenestra in the icthyosaurs has become closed up by the 

 posterior obtrusion of the orbit, as was suggested by McGregor. 

 That this was really the case, however, has by no means been 

 proven. We can conceive of an early diapsid stem without 

 lateral fenestration (as is indeed the case, in Procolophoii], from 

 which the ichthyosaurs might have been derived. Certainly the 

 Ichthyosauria are the most primitive of reptiles, save the coty- 

 losaurs, in so far as the lateral temporal region is concerned. The 

 squamosal and postfrontal here form the outer boundary of the 

 supratemporal vacuity, while the large prosquamosal is intercal- 

 ated between the squamosal, postorbital and quadratojugal. If 

 the ichthyosaurs originally had a latero-temporal fenestra it was 

 situated, in all probability, above the prosquamosal. (Fig. 17.) 





FIG. II. Pareiasaurits, after Seeley. 



Next to the ichthyosaurs, the most primitive of the diapsid 

 types, as urged by Broom and Osborn, seems to be Procolophon. 

 (Fig. 4). Here there is a peculiar arrangement of the bones of 

 posterior region. A slender one by the side of the frontal and 

 parietal, above the orbital opening, must be the postfrontal ; 

 while another bone articulating with the parietal and jugal, situ- 

 ated below and behind the eye cavity, is as certainly the post- 

 orbital. The orbit, we may conclude, has been extended back- 

 ward, not by the closing up of the laterotemporal vacuity, but by 

 the absorption of the postfrontal and postorbital, so as to come 

 into contact with the parietal between them. But the orbit does 

 not include the supratemporal vacuity, since neither the post- 

 orbital nor the squamosal helps form its outer boundary. There 



