212 BRITISH POSSIL REPTILES. 



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, (' Crocodilia,' PI. 31,) 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, « i, 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, (' Lacertilia,' PI. 8, fig. 3); 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 (PI. 31) 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 Pohjptychodon, 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 modification 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 Polyptychodo7i continuus, discovered by Mr. Bensted in his quarry 

 near Maidstone. 



