152 MARINE REPTILES OF THE OXFORD CLAY. 



region. The ventral face of the basisphenoid is almost completely concealed by the 

 adherent lyarasjjhenoid ijias.), the pnstero-external angle of which may perhaps take 

 part in the foimation of the facets (j>t-f^) for the processes of the pterygoids. The 

 ventral face of the posterior part of the bone is covered with irregular rugosities ; 

 further forwards it becomes quite smooth. From its posterior end the bone narrows 

 gradually till just in front of the anterior end of the basisphenoid, when it suddenly 

 widens out, the lateral borders of the widened portion bearing oblique surfaces {pt.f.^) 

 for union with the inner edge of the pterygoids. Anteriorly it terminates abruptly in 

 a thin, sharp, and somewhat concave border. The free portion anterior to the basi- 

 sphenoid is thin, but is strengthened by the presence of a pair of blunt ridges on the 

 upper surface on either side of the middle line, which is marked by a longitudinal 

 groove. The form and relations of the parasphenoid are shown in text-figs. 73, 74. 



The parietals are not well preserved ; it can be seen that the sagittal crest was short 

 and was high anteriorly. Posteriorly the bones send off short stout lateral processes 

 which unite with, and are to some extent overlapped by, the squamosals, while 

 anteriorly they widen out and are separated in the middle line by the large parietal 

 foramen, of which they form nearly the whole border. Their anterior edges unite 

 internally with the posterior ends of the frontals, which close the front of the pineal 

 foramen, and externally to these there is a broad sutural surface which, judging from 

 the structure in Mvrcenosaurus, united with the postfrontals. On the ventral surface 

 of the united parietals there is posteriorly a broad facet, looking backwards and 

 downwards, for union with the supraoccipital. In front of this the cranial surface of 

 the bones is at first nearly flat, becoming more and more concave forwards as it passes 

 into the posterior wall of the pineal opening, which is bordered laterally by strong 

 ridges. On the posterior border of the anterior expansion of the parietals there is a 

 pit-like facet, probably for the reception of the upper end of the columella cranii. 



The frontals, so far as they are preserved, are much like those of Muroenosaurus. 

 The nasals are unknown, and indeed the Avhole of the bones of the upper surface of 

 the skull are badly preserved or wanting altogether; it appears, however, that the 

 external nasal opening was large. 



The maxillce each bear fifteen teeth, of which the third and fourth are the largest . 

 behind them there is a gradual decrease in size to the hinder end of the series. The 

 facial (suborbital) plate of the bone is large, thin, and concave superiorly. The 

 palatal portions of the bone are small. The sutural surface for union with the 

 in-emaxillae is straight and nearly at right angles to the long axis of the skull. 



lYie premaonlloi^Ye almost triangular in outline and are prolonged back in the mid- 

 dorsal line into short facial processes. The facial surface is covered with pits and 

 strong ridges. Each bears five teeth — the first small, the second, third, and fourth 

 large, the fifth again small. 



The vomers are closely united into a triangular plate of bone, and their upper surface 



