84 MAEmE EEPTILES OF THE OXFOED CLAI. 



upper half of the foramen magnum. P'rom the upper border of the arch a sharp 

 ridge-like process (r.) projects downwards into the foramen. The ventral ends of the 

 n are greatly expanded and terminate in a roughly triangular surface looking 

 downwards, for union with the exoccipital and opisthotic {exo.op.f.), and a large surface 

 looking forwards and downwards for union with the pro-otic (pro./.); this latter facet 

 bearing near its inner border tw^o foramina united by a nearly closed channel which 

 probably lodged part of the posterior vertical semicircular canal {p.v.c). The summit 

 of the bone is occupied by a flattened and roughened surface, for union with the united 

 parietals [par./.), on the ventral face of which there is a corresponding slightly 

 concave facet. 



No good specimen of the iwo-otic has been found, but in one instance (No. IS, 

 R. 2861) it is preserved on one side, still united with the supraoccipital, but crushed 

 inwards. It appears to have been roughly triangular in outline, each of the angles 

 being truncated. Its upper anterior edge is broadly rounded ; behind this it unites by 

 a large surface with the supraoccipital and below this with the opisthotic, forming 

 ■svith these bones the usual triradiate suture ; at its lower anterior end there is a large 

 articular facet, perhaps for union with the basisphenoid. The inner face of the bone 

 is hollowed into a deep fossa, from which two channels diverge about at right angles ; 

 these probably lodged portions of the anterior vertical and the horizontal semi- 

 circular canals. 



The ])arietals {par., PI. III. fig. 2a; PI. TI. fig. 1 ; text-fig. 46) form the whole 

 of the cranial roof behind the parietal foramen ; they seem to fuse completely with one 

 another, though traces of their original separation can be seen, particularly on the 

 occipital surface, where a slight vertical ridge marks the line of union. They unite 

 with the supraoccipital by a rounded and slightly concave surface, and above this they 

 rise for some distance, forming a nearly vertical occipital surface, concave from side to 

 side. In this region the bones are produced laterally into short stout triangular 

 squamosal processes, which are completely overlapped along their upper borders by 

 the parietal processes of the squamosals, which meet in the middle line and unite 

 closely with one another, forming the actual vertex of the occipital surface of the 

 skull. In front of the squamosal processes the upper part of the parietals narrows into 

 a high sagittal crest, which extends forwards to within tw-o or three centimetres of the 

 pineal foramen {p.f.). Here the bones widen out considerably and enclose the large 

 foramen, uniting immediately in front of it with the frontals by a complex suture 

 running nearly transversely. In some cases this suture seems to cross the front of the 

 pineal foramen, in which case the frontals take a small share in the formation of the 

 anterior border of that opening. Externally the anterior expansion of the parietal 

 joins the postfrontal {po.f.) in a nearly longitudinal suture. Beneath the sagittal 

 crest the parietals widen out to form the cranial roof, their greatest width being about 

 opposite the hinder end of the pineal foramen, where on the ventral surface in one 



