150 MARINE REPTILES OE THE OXFORD CLAY. 



The united parietals (par.) form the posterior portion of the crest between the 

 temporal fossae, and anteriorly they are overlapped to a considerable extent by tbe 

 frontals ; the portion of the sagittal ridge formed by the parietals is usually much 

 narrower than the frontal portion, and may have a groove along its summit. Posteriorly, 

 the bone widens out and its dorsal surface usually forms a small triangular area. The 

 occipital surface is nearly vertical with a strong median ridge. The lower edges unite 

 with the nearly transverse upper border of the supraoccipital, except for a short 

 distance on either side, where a pair of foramina separate the bones. Laterally, the 

 parietals slope away and are prolonged outwards into a pair of processes which overlie 

 the upper surface of the inner arm of the squamosals (sq.) and extend some distance 

 down into the temporal fossse. 



In the cranial region the parietals unite in front with the alisphenoid (al.s.) and 

 behind with the pro-otic (pr.o.), which is separated from the alisphenoid by a nearly 

 vertical suture. At the point of union of the squamosal, parietal, and pro-otic there 

 is a foramen from which a groove runs out on the posterior wall of the temporal fossa, 

 terminating in another foramen between the squamosal and probably the quadrate ; 

 these foramina probably transmitted a blood-vessel. 



The alisphenoid (al.s., text- rigs. 55, 56), as already noted, unites above with the parietal, 

 the suture with which is slightly convex upwards. At its anterior end the alisphenoid 

 seems to be continued as a narrow strip along the edge of that part of the parietal 

 which in the crushed skulls seems to help in forming the floor of the anterior part of 

 the temporal fossa ; in the natural condition this portion of the parietal and alisphenoid 

 would probably slope more downwards and outwards, forming the upper part of the 

 side walls of the cranium. Posteriorly, the alisphenoid joins the pro-otic (pr.o.) in a 

 straight suture, running nearly vertically downwards to the foramen for the fifth nerve, 

 the upper edge of which is formed by a notch on the lower border of the alisphenoid 

 and pro-otic at their junction with one another; this foramen may have been closed 

 below by the inner extension of the quadrate. 



The bone which is here regarded as the pro-otic (text-fig. 55, pr.o.) unites above with 

 the parietal and posteriorly forms the lower border of the vascular foramen, the upper 

 edge of which is formed by the squamosal and parietal; behind the foramen there is a 

 short union with the squamosal. Its lower border at its junction with the alisphenoid 

 is notched by the foramen for the fifth nerve, as described above; behind this it unites 

 with the inner extension of the quadrate (q.). 



The squamosal (sq.) is an L-shaped bone consisting of a stout anterior process 

 uniting with the postfrontal (postorbital) and an inwardly directed process uniting below 

 with the quudrate and exoccipital and overlapped at its inner end by the lateral process 

 of the parietal; below, it forms part of the border of the vascular foramen mentioned, 

 and has a short union with the pro-otic. The inner faces of the two arms of the 



