12 DR. R. W. GIBBES ON THE 



tout le reste est bien presde I'etre, du moins pour ce qui regarde les caracteres 

 essentiels ; aussi n'ai je point eu de peine a reconnaitre a classer les vertebres quand 

 une fois j'ai bien connu la tete."* 



There is, however, an exception in the case of Megalosaurus and Geosaurus. Von 

 Meyer says of the former, " that notwithstanding the similiarity of the teeth with 

 those of Geosaurus, the structure of the rest of the skeleton differs verj'- considerably. 

 Among the bones yet found some remind us of the Crocodile, others of the Monitor, 

 while others have not been sufficiently determined. The beak appears to have been 

 straight and elongated ; the width of the vertebrae at present discovered, exceeds 

 their length by a third. They are contracted in the middle, as in Streptospondijlus ; 

 both surfaces are plane. "f And he again observes of them, that "the teeth are very 

 similar, while they have nothing else in common." Although, in the recent croco- 

 diles, the vertebra; are convex on one surface and concave on the other, still there 

 are some fossil species which are plane or concave on both surfaces, and thus re- 

 semble those of Cetacea. 



"In the recent crocodile," says Dr. Mantell, "the vertebrce are convex posteriorly, 

 and concave anteriorly ; but those from Tilgate, like the vertebrse of the crocodile of 

 Caen, and of one of the species of Havre, are, with but few exceptions, slightly con- 

 cave at both extremities "J la Megalosaurus both surfaces are slightly concave; in 

 Iguanodon, they are almost flat on one side and slightly depressed on the other ; in 

 Geosaurus slightly concave ; in Streptospondijlus " the vertebrse are distinguished by 

 the very remarkable peculiarity, that their anterior surface is convex, and the hinder 

 concave ; consequently the reverse of those of the Crocodiles, and formed like the cer- 

 vical vertebra? of the whole order of terrestrial mammalia."^ On this peculiarity the 

 genus Streptospondijlus is founded. Cuvier also says : 



" Mais il y a des vertebres de grands reptiles, qui ressemblent si fort a celles de 

 certains dauphins qu'il est facile de s'y tromper ; toujours faudrait-il examiner leur 

 l)osition avec beaucoup de soin, et voir s'il n'y aurait point, a cet egard, de difference 

 entre ces os de cetaces et ceux de reptiles." || 



For several varieties of vertebrte of fossil Saurians differing from the recent species, 

 I refer to Dr. Mantell's work on the Fossils of Tilgate Forest. These remarks and 

 references are necessary in relation to the description of my later specimen, which 

 possesses characters belonging to the Cetacea and Sauria, though the former pre- 

 dominate. 



In the lower maxilla (PI. III., figs. 4 and 6) there are marked differences from the 

 former, (PL III., figs. 1 and 3.) The external surface is convex, the inner hollowed, 



* Cuvier, Art. Saiiriens, Vol. V., p. 326, 



t On Ihe stnictnre of Fossil Saurians, in Chaileswoilh's Magazine of Nalnfnl Mistoiy, Vol. I., p. 341. 



J Geology of South East of England, p. 263. 



§ H. Von Meyer, ut supra, p. 352. 



II Letter to G. Mantell, Geology of South East of England, p. 2S2. 



