264 THEORY OF THE EARTH. 



before, and concave behind. Its tail was deep 

 and flat, and formed a large vertical oar (or or- 

 gan of swimming). * Mr Conybeare has re- 

 cently proposed to name it Mosasaurus. 



The clays and lignites which cover the upper 

 part of the chalk, I have only found to contain 

 crocodiles f ; and I have every reason to think 

 that the lignites which in Switzerland have af- 

 forded beaver and mastodon bones, belong to a 

 later epoch. Nor has it been at an earlier period 

 than that of the coarse limestone which rests up- 

 on these clays that I have begun to find bones of 

 mammifera ; and still do they all belong to ma- 

 rine mammifera, to dolphins of unknown species, 

 lamantins and morses. 



Among the dolphins, there is one, the muzzle 

 of which, more elongated than that of any known 

 species, has the lower jaw united in a conside- 

 rable part of its length, nearly as in a gavial. It 

 was found near Dax by the late president of 

 Bordaj. 



Another species, from the cliffs of the Depart- 

 ment de 1'Orne, has the muzzle also long, but 

 somewhat differently shaped . 



The entire genus of lamantins is at the present 



Researches, vol. v. part iv. p. 310, et seq. 



t Ibid. p. 163. J Ibid. p. 316. P. 317. 



