STENEOSAUEUS OBTUSIDENS. 131 



exist, the possibility that the specimens upon which the latter is based may be very old 

 and large individuals of St. durobrivensis cannot be entirely ignored. 



The type specimen is a very large and presumably old individual, the skull of which, 

 in its general proportions, is similar to that of Steneosaurus durobrivensis, and if it 

 were not for the peculiarity of the teeth, might almost have been referred to that 

 species ; but since, so far as I am aware, there is no evidence of considerable change in 

 tooth-form during the life of these animals it is better to separate the present species, 

 especially as this distinction is supported by some other characters. In the skull 

 (PI. VII. fig. 1) the rostrum is very thick and massive, perhaps more so than in the 

 other species. The orbits are large and oval ; the frontal terminates anteriorly in an 

 obtuse ano-le about on a level with the anterior border of the orbits : its surface is 

 ornamented by a number of obscure ridges radiating from the centre. There is 

 a small slit-like antorbital foramen about 7 cm. in front of the orbit. The rostrum 

 has its upper surface strongly convex from side to side, and at its anterior end 

 seems to have curved a little upwards ; in front of the orbits it narrows very 

 gradually to a point about 1G cm. behind its anterior end, where it is only 7*3 cm. 

 wide ; in front of this the premaxillary region is expanded. The total number 

 of teeth in the upper jaw cannot be made out. In this skull the length of the 

 preorbital region is about 51 per cent, of the whole ; in St. leedsi it is about 73 per 

 cent, and in St. durobrivensis about the same as in the present species, in which, 

 however, the rostrum seems to narrow rather more gradually. 



The mandible is very massively constructed. The symphysial region, which extends 

 back to about the level of the twenty-fourth tooth, occupies about 42 per cent, of the 

 total length, is flattened ventrally, and has a considerable anterior expansion, greatest 

 at the fourth tooth. In St. leedsi the symphysis occupies about 58 per cent, of the 

 total length of the mandible, in St. hulkei about 40 per cent., and in St. durobrivensis 

 44 per cent., these two species in this respect being very similar to the present 

 one. 



The lower teeth are about 28 in number on each side. The first two are small, 

 then follow two large teeth closely crowded together, behind these again there is a 

 small tooth followed by an increase in size till near the hinder end of the series. 

 One peculiarity in the dentition of the type specimen, is that about half the teeth in 

 the lower jaw bite into deep pit-like sockets in the maxilla, the walls of these sockets 

 forming slight prominences along the outer alveolar border of the upper jaw. It is, 

 however, uncertain whether this remarkable character is normal, and it seems that the 

 condition may have been produced by great pressure which forced the points of the 

 lower teeth into the maxillae while the bone was in the semi-plastic state which 

 many of the specimens in the Oxford Clay seem to have passed through. 



The structure and proportions of the cervical, dorsal, and caudal vertebrae seem to 

 be very similar to those of the corresponding vertebrae in St. durobrivensis, but in the 



s2 



