68 FOSSIL REPTILIA OF THE 



In the instructive Collection of Thomas Charles, Esq., of Maidstone, is part of a 

 single digit of the paddle, T. XVII. fig. 2, of apparently the same species of Plesio- 

 saurus. It includes three phalanges, and part of a displaced small phalanx of an 

 adjoining digit. In comparison with the more perfect paddle in Mrs. Smith's collec- 

 tion, I regard the phalanges in the present specimen as being the third, fourth, and 

 fifth of their digit. 



Genus, Ichthyosaurus. 



If the investigation of the fossil remains of the Chalk-beds had been undertaken 

 by the Comparative Anatomist, without previous knowledge of the fossils of the lower 

 secondary formations, he would have perceived in the teeth which form the subjects 

 of T. XXIV, characters not only specifically, but generically, distinct from any of the 

 teeth that have been previously described and figured in the present monograph. The 

 thick conical crown covered by enamel, raised into numerous longitudinal ridges, would 

 have offered, it is true, a repetition of the general character of that of the teeth of Poh/pty- 

 cJwdon ; but the continued expansion of the base or fang of the tooth, and the coarser 

 longitudinal ridges and grooves with which most of the surface of that part is sculp- 

 tured, would be a peculiarity distinguishing the present from any of the previously 

 noted teeth from the Cretaceous or Tertiary series, and still more so from the teeth of 

 any known existing Reptile. It is only, indeed, those of the largest Crocodiles or 

 Alligators that can compete with the present fossil teeth from the Chalk-formations in 

 point of size ; and the crowns of these, as in the teeth of the Polyjotychodon, differ from 

 the teeth of the Crocodilia in the absence of the two opposite ridges, forming or indi- 

 cating the edges of the crown ; whilst their base also differs from that of the 

 Crocodile's tooth in the structure above defined, — a difference which becomes more 

 manifest when a section of that part is made, demonstrating that the expanse of the 

 fang is due to the unusual thickness of the osseous external crust called ' cement.' 

 The Anatomist, I say, would be justified in deducing from these characters the generic 

 distinction of the Reptile to which they had belonged, but he could have formed no 

 suspicion of the truly extraordinary modifications of the entire reptilian organisation that 

 had been associated with such generic modifications of the teeth. Such fossil teeth, 

 having a conical, enameled, and commonly striated crown, offering a considerable 

 range of variation in its proportions, and supported by an expanded, usually solid, and 

 coarsely-grooved fang, covered \>y a thick coat of cement, have been recognised, since 

 the publication of Sir Everard Home's Paper ' On the Remains of an Animal linking the 

 class of Fishes to that of the Crocodile,' published in the Philosophical Transactions 

 for 1814, as belonging to that genus of animal to which Home gave the name of 

 Proteosaurus, but to which Naturalists have concurred in applying the more classically 



