CRETACEOUS FORMATIONS. 59 



(T. X, fig. 5 ;) to Coniosaurus, (T. IX, fig. 14« ;) to Mosasaurus, (ib., fig. 1 ;) to Leiodon, 

 (T. IX^, fig. 1 ;) and to Polyptychodon, (T. XI and T. XIV,)— we now, for the first 

 time, in our progressive researches, descending through the strata which indicate 

 the changes which the part of the earth's surface forming England has undergone, 

 meet with teeth of different and peculiar type, remarkable, viz., for their length and 

 slenderness, and with a circular transverse section, not subcompressed or with opposite 

 trenchant margins, as in the Gavials of the Tertiary deposits. The tooth represented 

 of the natural size in T. IX, fig. 8, is a good example of one of those of the form in 

 question. Its enamelled crown, if entire, would exceed an inch and a half in length, 

 yet it is but half an inch in diameter at its base ; the crown is slightly curved 

 and tapers gradually to a point ; the enamel presents some slender but well-defined 

 longitudinal ridges of different lengths, and none of them extending to the apex. The 

 fang or root is cylindrical, smooth, and covered by a thin cement. The tooth above 

 described was obtained from the Scaddlescombe Chalk-pit, near Lewes, Sussex. 



A similar specimen, rather more fractured, T. IX, fig. 10, was found in a Chalk-pit 

 at Southeram, Sussex. 



A smaller tooth, (T. IX, fig. 9,) of the same type, but with more numerous longi- 

 tudinal ridges, seems to indicate a different species. This specimen was also found 

 at Southeram. 



If satisfactory and abundant evidence of the nature of the extinct reptile to which 

 the above-described teeth belong had not been obtained from Secondary Formations of 

 a more ancient date than the cretaceous ones, the Comparative Anatomist would have 

 inferred, and correctly, the generic distinction of the Reptile to which they belonged ; 

 but he could-have had no suspicion of the truly extraordinary nature of the animal, the 

 entire race of which, after flourishing under a variety of specific forms from the epochs 

 of the Muschelkalk and Lias, finally perished at the time of the deposition of the 

 Chalk. 



The anatomical description of the Plesiosaimis, discovered and restored by 

 Conybeare and De la Beche, will be reserved for the Monograph descriptive 

 of the fossil Reptiles of the formations in which its remains are most abun- 

 dant ; and I shall here limit myself to quoting the brief but graphic definition of 

 it which Dr. Buckland has given in his interesting and instructive 'Bridgewater 

 Treatise :' — " To the head of a Lizard it united the teeth of a Crocodile ; a neck of 

 enormous length, resembling the body of a Serpent ; a trunk and tail having the 

 proportions of an ordinary quadruped ; the ribs of a Chameleon ; and the paddles of a 

 Whale. Such are the strange combinations of form and structure in the Plesiosaurus," 

 (p. 102.) I may add, that of all existing Reptiles the Chelonians make the nearest 

 approach to the present remarkable extinct genus in the length and flexibility of the 

 neck, in the size of the true body of the atlas, which resumes its nqrrnal relations 

 with the neural arch of that vertebra in Chelys and C/ielodina, as in Plesiosaurus ; 



