52 BRITISH FOSSIL REPTILES. 



surface, at its middle third, for the presphenoid (9), and a smooth emargiuation on each 

 side forming the hind border of the sphenopterygoid or ' interpterygoid ' vacuities («, «).^ 

 The hinder half of the under surface of the basispheuoid presents shallow rough depres- 

 sions and feeble risings for muscular attachments, and, like the basioccipital, it is 

 imperforate. Of the alispheuoids I have been unable to determine more than their 

 presence and their small size. The side vk^alls of the brain-case proper seem to have been 

 mainly cartilaginous. 



The parietals (7) in most Ichthyosaurian skulls retain their median (sagittal) suture 

 (Tab. XIX, fig. 1, 7), which usually opens out anteriorly to form the hind end of the 

 ' foramen parietale ' or fronto-parietal fontanelle' (/), the chief part, or whole, of which is 

 bounded by the frontals (11). 



The upper surface of the parietals seems, by reason of the aspect of the occipital por- 

 tion, to be divided by a ridge (r) extending from the mastoids (8"), and continued upon 

 the parietals to their mid-suture, into an anterior (7) and posterior (/') surface. This masto- 

 parietal ridge (s" r) properly bounds, above, the occipital surface, to which the parietals 

 thus contribute about a fifth part of their length above the superoccipital bone (Tab. XXII, 

 fig. 1, 7'). Anterior to this ridge each parietal slopes to the temporal fossa (Tab. XIX, 

 fig. 1, t), the parietal surface being divided by a low longitudinal rising continued forward 

 from a posterior convexity into two facets, both of which are concave across. The dividing 

 ridge is overlapped by a postero-mesial angle of the postfrontal (12), between which and the 

 frontal a narrow forward continuation (7") of the parietal is exposed, which overlaps the 

 hind part of the frontal. The margins of the sagittal suture usually rise into a low ridge, 

 which is continued upon the occipital part (;') of the parietal. Of this part (Tab. XXII, 

 fig. 1, 7') the surface on each side of the mesial ridge is feebly concave, almost flat, laterally 

 overlapped by the mastoids (s") ; it seems to rest upon, without sutural junction with, 

 the superoccipital (3). The postero-lateral extension of the parietal (ib., fig. 1, 7x) 

 curves down beneath the mastoid (s) to within a third part of the lower end of that bone, 

 contributing therewith to the upper and lateral parts of the broad occipital surface. 



The mastoid (8,8', 8") is a large and strong triradiate bone, the rays inclining forward 

 from the outwardly obtuse centre or body forming the prominence at each postero-lateral 

 angle of the skull. The upper and inner ray or branch (Tab. XIX, fig. 1, 8'') is three- 

 sided, one facet looking upward, a second backward, the third forward and outward, contri- 

 buting, with the parietal, to the outer and hinder wall of the temporal fossa (t). The angle 

 at which the anterior joins the superior facet is continued forward upon the ridge (s'V), 

 dividing the hinder and upper facets of the parietal bone. The lower mastoid branch 



1 'Anatomy of Vertebrates,' vol. i, p. 157, fig. 9S, B, s. 



2 "This foramen, or ' fontanelle,' is common in the Tnassic Reptilia. It is described and figured in 

 Galesaurus, Petrophryne, Dicynodon, Ptyehognathus, Oudenodon, Kisticephalus, and Procolophon ; iu the 

 latter it is large. It is wholly ' parietal ' in Kisticephalus and Ptyehognathus, in which it is placed far 

 back." — ' Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the British 

 Museum,' 4to, 1876. 



