58 FOSSIL REPTILIA OF THE 



One or two of the long ridges of this tooth are more than usually prominent, and most 

 of the shorter ones are fainter than usual; but I cannot regard those differences in any 

 other light than as individual varieties. The pulp-cavity at the base of the tooth, filled 

 up in the specimen by the white chalk, appears to have been unusually large, as if the 

 specimen had been in an incompletely developed state. If this were the case, it must 

 have come from a very large specimen of the present species of extinct reptile. 



To such a specimen must have belonged the anterior end of the left ramus of the 

 lower jaw, (T. XVI,) discovered in the Burham Chalk-pit, in Kent, and now in the 

 choice and instructive Collection of J. Toulmin Smith, Esq. The fragment is upwards 

 of a foot in length, but contains only three alveoli, and corresponded, probably, to the 

 premaxillary part of the upper jaw of the same animal. The first socket, « l, is nearly 

 three inches from the fractured end of the jaw, and two inches from the larger socket, 

 s 2, behind it ; the third socket, s 3, is closer to the second. These are filled up by the 

 chalk, the teeth having fallen out. The outer surface of the jaw is convex and promi- 

 nent ; a solid mass of the bone extends horizontally inwards from the anterior socket, 

 to form the symphysis, which seems to have been ossified, with the opposite ramus. 

 The substance of the bone has the same coarse cancellous tissue as that of the portion 

 of the smaller jaw of Polyptychodon, (T. X, fig. 7); and, as it shows a similar inequality 

 in the intervals of the alveoli, it may be concluded to belong to the same genus, if not 

 species, of extinct Crocodilian reptile. The present fragment indicates an individual 

 as large as the great Mosasaurus, the skull of which was discovered in the Maestricht 

 Chalk. 



Fine specimens of crowns of the teeth of both species of Polyptychodon, T. X, figs. 

 8 and 9, have been obtained by James Carter, Esq., M.R.C.S., of Cambridge, from the 

 Upper Green-sand near that town, and also at Horn-sea, in the same county. These 

 specimens present a darker colour than those of the chalk, by reason of the modifi- 

 cation of their matrix. The ridges are remarkably well defined on the enamel ; the 

 dentine presents the same well-marked division into layers, cone within cone, as in 

 the Chalk specimens, and that from the Shanklin sand near Maidstone. The crown 

 of one of the specimens of the Polyptychodon interruptus from the Cambridge Green- 

 sand, equals in size that of the Polyptychodon continuus, discovered by Mr. Bensted in 

 his quarry near Maidstone. 



Order. ENALIOSAUBIA. 



Genus. — Plesiosaurus, Conybeare. 



Besides the teeth which, according to their form and structure, were referable to 

 the different genera and species of Reptilia above described, — viz. to Raphiosaurus, 



