CRETACEOUS FORMATIONS. 47 



chodon, the interspaces between the longer ridges widening as they approach the 

 apex. The teeth of the Polyptychodon never offer any approach to opposite trenchant 

 edges of the crown : but this part, presenting throughout its extent a transverse section 

 of an almost circular form, (T. XI, fig. 7, Tab. XIV, fig. 3,) is slightly and regularly 

 bent lengthwise, and is invested with a moderately thick layer of true enamel, of 

 which substance the ridges are wholly composed, the surface of the outermost layer 

 of dentine being quite smooth, (Tab. XIV, fig. 4). The teeth of the Polyptychodon 

 may be distinguished at once from those of the Mosasaurus or Pliosaurus by the 

 absence of the less convex, or almost flattened facet of the crown, which is divided by 

 strong ridges from the remainder of the crown. 



Polyptychodon continuus, Owen. Tab. XIV, figs. 4, 5, 6. 



'Odontography,' vol, ii, p. 19. 



The first evidence of this species was a single tooth, which was discovered by 

 W. H. Bensted, Esq., of Rock Hall, near Maidstone, September 16th, 1834, in what 

 is called the ' Trigonia-stratum ' of Shanklin Sand, in the Kentish Rag Quarries near 

 that town, this stratum being a member of the Lower Green-sand Formation. The 

 tooth in question (T. XIV, figs. 5 and 6) has a crown upwards of three inches in length, 

 and one inch four lines in diameter across its base. The compact dentine has been 

 partially resolved by decomposition into a series of superimposed thin hollow cones, 

 fig. 6, and the short and wide conical pulp-cavity is confined to the base, and 

 beginning of the fang, which has been broken away. The cavity of the crown of the 

 tooth in Hypsodgn would seem to have been always much larger, as it is in many other 

 predatory fishes in which the teeth are more rapidly shed and renewed than in the 

 Crocodilian Reptiles. In the Collection of Henry Catt, Esq., of Brighton, is preserved 

 the crown of a nearly equally fine specimen of the Polyptychodon continuus, from the 

 Chalk of Sussex : this specimen is figured of the natural size in T. XIV, fig. 4. A portion 

 of the ridged enamel has scaled off, exposing the smooth surface of the dentine which 

 it protected. The teeth of this species of Polyptychodon differ from those supposed to 

 have belonged to Poilcilopleuron, in the ridges of the crown being more numerous and 

 close set, and in the transverse section being circular instead of elliptical. 



Gigantic Fossil Saurian from the Lower Green-sand at Hythe. 



Tabs. XII and XIII. 



'Proceedings of the Geological Society of London, June 16th, 1841.' 



I propose to describe these remarkable and highly interesting fossils under the 

 present section, on account of the identity of the Formation in which they were dis- 

 covered, with that of the tooth of Polyptychodon continuus above described, and because 



