28 MAEINE EEPTILES OP THE OXFOED CLAT. 



be made out clearly, but it seems to have united behind in a complex suture with 

 the squamosal; anteriorly it joins the frontal, prefrontal, and perhaps the parietal. 

 In its narrow middle region it forms the upper border of the orbit and the outer border 

 of the supratemporal fossa ; its orbital border is thin and sharp, and within this the bone 

 thickens gradually towards its middle to form a gently concave roof to the orbit. The 

 inner edge forming the border of the supratemporal fossa is thickened and rounded. 



The p'efrontnl consists of a strong rounded central portion, which is expanded above 

 to form the rim of the orbit. At the lower decurved end it interlocks with the upper 

 end of the lachrymal. On its upper surface it bears a strong ridge, which fits into a 

 corresponding groove on the overlapping nasal. Postero-internally it joins the frontal, 

 and externally its upper surface is extensively overlapped by the postfrontal. 



Th.e pterygoids (text-fig. 17) are very large bones, expanded posteriorly, and produced 

 anteriorly into long pointed processes. The broad posterior portion united by its 

 inner upper surface (is.pt.f.) with the basipterygoid processes of the basisphenoid, and 

 behind these also overlapped the ventral sui-face of that bone to a considerable extent 

 by a postero-internal expansion (m.p.). External to this expansion and separated from 

 it by a deep groove the dorsal surface of the bone bears a strong oblique crest-like ridge 

 {d.]}.), the upper surface of which, together with that of the postero-external process 

 (e.j).), is closely adherent to the inner face of the quadrate, the posterior half of which 

 it completely covers ; at its upper end it is itself overlapped for a short distance by the 

 quadrate process of the squamosal. Into the deep groove (g.st.) between the dorsal 

 and postero-external processes the lower border of the large stapes Hes, its outer end, as 

 already described, fitting against a facet of the inner face of the quadrate immediately 

 behind the pterygoid. In front of the posterior expansion the bone first narrows 

 into a sort of neck, of Avhich both the outer and inner borders are concave ; then it 

 expands into a broad flat plate, which gradually narrows forwards till, at eibout one-third 

 of its length from the anterior extremity, it passes forwards into an elongated 

 vertical plate forming the anterior third of the bone. This plate bears on both its 

 outer and inner surfaces elongated slightly ridged facets for union with adjoining 

 bones, that on the outer face being no doubt for the vomer, that on the inner for 

 the corresponding facet on the pterygoid of the opposite side. The posterior third of 

 the inner border {i.b.) of the portion of the bone in front of the union with the 

 basisphenoid is thickened and concave ; it forms the outer border of the inter- 

 pterygoid vacuity, which is divided in the middle line by the parasphenoid, the 

 anterior end of which thrusts itself between and unites with the pterygoids for some 

 distance {pas.f.) ; in this region the inner border of the pterygoids is nearly straight. 

 Anteriorly, as already mentioned, they are in contact with each other in the middle 

 line, and probably lay between the vomers. The thickened inner border of the 

 posterior part of the bone forms a ridge (?•.), which is continuous anteriorly with the 

 upper edge of the vertical intervomerine plate. 



