84 MARINE EEPTILES OF THE OXFORD CLAT. 



On the palatal surface (text-fig. 32) the maxillae meet in the middle line from their 

 anterior end to the anterior angle of the palatines. In front they usually send 

 forwards a tongue-like prolongation between the palatine processes of the premaxillae 

 as described above. Posteriorly they diverge and unite in suture with the palatines 

 for some distance, but at a point a little in front of the anterior edge of the orbit 

 they are separated from them by the anterior portion of the suborbital vacuity (s.o.v.). 

 The alveolar borders are nearly straight, being slightly concave in the posterior portion 

 only. The alveoli have well-developed borders and form slight projections on the 

 outer edge of the jaw. The number of alveoli varies much in the different species. 

 In S. leedsi, in which the greatest number occurs, there are thirty-eight maxillary teeth 

 on either side of the jaw. In all species the first maxillary tooth is small and is often 

 situated at a rather higher level than the rest. Behind this the teeth remain almost 

 equal in size till the last few, which gradually diminish. The palatine plates of the 

 maxillae are sharply separated from the alveolar border by a groove, on the inner side 

 of which they are sharply convex, then nearly flat to the middle line ; in some cases 

 the suture between them may run along the summit of a slight median ridge. Their 

 union with the palatines has already been referred to : at the suborbital vacuities, of 

 which they form the outer boundaries, they narrow to their posterior end, uniting 

 behind for a short distance with the transpalatines (f_p.), which behind them join 

 the jugals (j.). In the region of the palatines, the palatine plates of the maxillae are 

 somewhat inflated and convex from side to side. 



The palatines [pal.) meet one another in median suture on the palate. Anteriorly 

 they send forwards a narrow process between the maxillae, then, still united in suture 

 with the palatine plates of the maxillae, they widen out to the anterior angle of the 

 suborbital vacuity, of which they form the inner border. Here they narrow slightly 

 till a little in front of the opening of the internal nares, behind which they widen out 

 again and join the pterygoids. The ventral border of the internal nares (i.nar.) is 

 formed entirely by the secondary palatal plates of the palatines ; it is gently concave 

 and is somewhat thickened and irregularly notched, at least near the middle. The 

 roof and part of the sides of the narial passage are formed by the pterygoids (pt., 

 text-figs. 32, 33). The upper portion of the palatines, which should form the roof 

 of the nasal canal in front of the pterygoids, is wanting in all cases, so that it cannot 

 be seen whether or not they completely enclosed a portion of the canal, as described 

 by Deslongchamps *. The vomer also has not been seen in any of the specimens, so 

 that its form and relations to the surrounding bones cannot be described. 



The pterygoids (pt,, text-figs. 32-33) are very large bones with a great antero-posterior 

 extent. As already mentioned, they form the roof of the posterior narial opening, 

 the suture between them and the palatines crossing just behind that aperture on the 



* 'Notes Paleontologiques ' (1S67) p. 208. 



