ON BRITISH FOSSIL REPTILES. 65 



the magnificent remains from Market-Raisin*, would be equally applicable 

 to the short-necked gigantic Pliosaur from Shotover, and consequently lose its 

 value as a distinctive appellation. 



CROCODILIA. 



The remains of species of this order extend from the Eocene tertiary for- 

 mations as low down as the Oolite and Lias, and offer deviations from the 

 structure of the existing genera and species, which increase in degree and 

 amount as the strata containing the extinct species indicate periods more 

 remote from the present time. 



Not any of the species are identical with those now known to exist, and the 

 modifications of the' osseous structure, by which the extinct Crocodilians differ 

 both from the present races and from one another, are much greater than any 

 of those by which the skeletons of the existing species differ among themselves. 

 Not only do the form and proportions of the periphei'al parts, as of the jaws, the 

 teeth, and the locomotive extremities vary, but the spine, or central axis of the 

 skeleton, offers modifications of the articular surfaces of the component verte- 

 brae whicli are quite unknown in the Alligators, Crocodiles and Gavials of the 

 present epoch. In these existing species the anterior surface of the vertebral 

 centrum is concave, the posterior convex, except in the atlas and sacrum. But 

 besides this mode of junction, Cuvier has recognized in the Crocodilians of 

 the secondary formations two other types of vertebral structure : in one of 

 these the positions of the ball and socket are reversed ; in the other, and more 

 common modification, both articular surfaces of the vertebra are flat or slightly 

 concave. Remains of extinct Crocodilians, exhibiting all the three systems 

 of vertebral articulation, occur in English formations. The extinct species, 

 which agree with the existing Crocodilians in their vertebral characters, will 

 be first described. 



a. fVith concavo-convex Vertebrcs. 



Crocodilus Spenceri, Buckland. 



' Crocodile de Sheppj/,' Cuv. 



The most recent stratum in which I have met with the remains of extinct 

 Crocodiles in Great Britain is the Eocene deposit called th^ London clayf. 

 A third cervical vertebra from the Isle of Shcppy, is noticed by Cuvier as 

 being very similar to the corresponding bone in an existing Ci'ocodile, and as 

 having appertained to an individual of probably five feet in length. No other 

 part of this Eocene reptile is noticed in the last edition of the ' Ossemens Fos- 

 siles;]:.' A fine cranium is preserved in the British Museum; and Dr. Buck- 

 land § has figured a smaller but better preserved specimen of the Sheppy 

 Crocodile, in the collection of E. Spencer, Esq. I have examined both these 

 specimens, and have compared them with the skulls of the recent Crocodilians. 



In Mr. Spencer's fossil, the end of the snout, including the intermaxillaries 

 and nostrils, is broken off; the tympanic pedicles, pterygoid alas and occipital 

 tubercle, and the crown of the teeth are also wanting. The principal and 

 most characteristic differences which the Crocodilus Spenceri presents in 



* Odontography, p. 283. 



t Cuvier makes mention of the calcancum of a Crocodile in the collection of M. G. A. 

 Dcluc, said to liavc been discovered at IJrnntford in the year 1791, associated witli the re- 

 mains of tlic Mammoth, Hippopotamus, Rhinoceros and i)cer, and wliich bore incontcstaI)le 

 marlis of a distinct species, lie observes that if this hone; lunl not been tvansporti'd to its 

 present situation, with the debris of otlicr strata, it would be the most recent of the remains 

 of tlie genus of Crocodile. — Loc. cit., p. 33G, vol. ix. 



X 8vo, 183G,vol. ix. ]). 327. 



§ ISridgewatcr Treatise, vol. i. p. 251, pi. xxv. fig. 1. 

 184.1. F 



