PELONEUSTES. 39 



anterior vertical and the horizontal semicircular canals. The outer surface is smooth 

 and convex in all directions, except at the base of the anterior process. 



From the account here given it will be seen that the arrangement of the bones 

 containing the auditory apparatus is much as in other reptiles, and is especially closely 

 comparable with that found in some Chelonians *. 



The basisphenoid is not well preserved in any specimen ; its ventral surface is 

 concealed by the overlap of the pterygoids and parasphenoid. Posteriorly it united 

 with the basioccipital by a flat and nearly vertical surface. The hinder half of the 

 upper surface is nearly flat, while the anterior portion is occupied by the large and 

 deep sella turcica, into the postero-external angles of which the internal carotids 

 open. Posteriorly the fossa is bounded above by two stout processes projecting over it 

 and also outwards ; the upper edge of these outer projections bears a facet for union 

 with some element, and although, owing to the displacement of the parts in all the 

 specimens, it is not possible to be certain what that element is, it seems probable 

 that it is the anterior process of the prootic. The sides of the basisphenoid beneath 

 the sella turcica bear strong lateral processes for union with the pterygoids. 



The parasphenoid (pas., PI. IV. fig. 1) is not well preserved in any of the 

 specimens. It overlaps the basisphenoid posteriorly ; in front of this it narrows so 

 as to be nearly as deep as wide, and divides the posterior interpterygoid vacuity into 

 two openings. Anteriorly it again becomes flattened and is thrust for some distance 

 between the palatine plates of the pterygoids, which, however, a little in front of the 

 vacuities, meet beneath it so that its full extent cannot be seen from the palatal 

 surface. The upper surface of the parasphenoid in the neighbourhood of the vacuities 

 and in front of them is marked by a longitudinal groove, of which the floor is 

 roughened ; this may indicate the presence of a vertical median plate of cartilage in 

 this region. 



The farietals (par., PL IV. fig. 2 ; text-fig. 13) form a high crest between the 

 temporal fossa? ; they descend considerably on the side of the brain-case and their 

 lower edge unites anteriorly with the upper edge of the epipterygoid (columella cranii, 

 col.), which is very well developed (see below). Posteriorly they unite below with 

 the facet on the upper border of the supraoccipital and send out lateral processes 

 which unite with the upper (parietal) branches of the squamosals to form the post- 

 temporal bars. What share each of the two bones takes in their formation cannot 

 be determined, the sutures being obliterated, but it seems probable that the 

 squamosals extend up along the upper edge of the parietals to within a short distance 

 of the middle line. The temporal bars thus formed are curved, the convexity being 

 directed upwards and forwards ; their posterior face is convex from above downwards, 



* Siebenrock, " Das Kopfskelet der Schildkroten," Sitzungsb. math.-naturwiss. 01. k. Akad. Wiss. Wien, 

 vol. cvi. pt. 1 (1897), p. 245, pi. iii. 



