KIMMERIDGIAN PLIOSAURS. 157 



Between the fifth and the sixth alveolus is a diastema of about 8 lines ; the long 

 diameter of the sixth alveolus is 1 inch 10 lines. An interval of 5 lines divides 

 it from the seventh socket. The succeeding ones are closer together : they 

 gradually increase in size to the twelfth or thirteenth, but do not obtain the size of 

 those opposed to them above ; they then gradually decrease in size and depth to a 

 diameter of about half an inch. 



The summits of crowns of successional teeth protrude from fossEe at the inner 

 and back part of the anterior alveoli. The crown of a more advanced successional 

 tooth projects into the bottom of the socket of the third and fifth of the 

 symphysial series : these teeth show the characters of the genus Pliosaimis. 



The inter-alveolar part of the "symphysis mandibulfe" forms a median 

 longitudinal rising, less convex or ridge-like than the one on the palate above. 

 FossEe are discernible on the inner side of the mandibular alveoli, but less marked 

 in the upper jaw. The apex of a successional tooth appears in two of these pits. 

 On the inner side of the posterior third of the mandibular ramus there is a wide 

 and deep channel between the surangular (29) and angular (30) elements ; and 

 this groove is continued forward indicative of the upper border of the splenial 

 (31) which extends along the inner side of the lower half of the dentary nearly to 

 the symphysis. The articular surface of the mandible (29), 7 inches is transverse, 

 and 5 inches in antero-posterior extent, is slightly concave transversely at the 

 inner three fourths of its extent, and then gently convex at the outer fourth ; it is 

 more concave from before backwards in the major part of its extent, but the 

 peripheral boundary is not entire. 



The sauropterygian affinities, as contradistinguished from the ichthypterygian, 

 are exemplified in the more complete and separate sockets of all the teeth, and in 

 the smaller projDortion contributed by the premaxillaries to their support and to 

 the formation of the upper jaw. 



The palato-nares of PUosaurus are more linear and approximate than in the 

 species of Plesiosanrus {PL Haivhinsii, PL XVI ; and PI. rostraius, PI. XIII, r, r) 

 in which they have been observed. 



Species — Pliosmtrus brachydeirus, Owen. 



In the Museum of Geology at Oxford are considerable proportions of the upper 

 and lower jaws of a PUosaurus from the Kimmeridge Clay at Market-Raisin.^ 

 The teeth, in proportions and arrangement, correspond so closely with those in 

 the specimen above described as induced me to surmise that they might belong to 

 the same species. The following are the difierences which I have noted between 



1 ' Second Eeport on British Fossil Eeptiles,' " Eeport of British Association," p. 61, 1841. 



