250 PICTURESQUE SKETCHES 



exhibit any peculiarities which demand consideration 

 here. They are not of large size, and they all be- 

 longed to the group inhabiting the sea. Their re- 

 mains, although not common, include several species. 



The gigantic reptile of the greensand called Polyp- 

 tychodon* is known only by such imperfect fragments 

 that we can scarcely do more at present than indi- 

 cate its vast proportions. The thigh-bone of this 

 animal was solid, and measured nearly four feet in 

 length. 



The remains of Ichthyosaurus and Pterodactyl in 

 the chalk are beyond question, and the specimens are 

 of great interest. The former is known by a very 

 beautiful specimen of the head with teeth, and the 

 latter by the head, and also several of the long bones. 



The bones of birds have been determined among 

 the fossil remains of the chalk ; and, although very 

 rare and in fragments, they seem to indicate, beyond 

 a doubt, the existence of a large species allied to the 

 albatross. 



No trace whatever of the existence of any land 

 quadruped, or of any of the whale tribe, has been 

 met with in the chalk, or in any of the associated or 

 contemporaneous beds. This entire absence of the 

 Mammalia cannot but be looked on as an interesting 

 and remarkable fact. 



The conditions of existence during the chalk and 

 the cretaceous beds generally were certainly different 

 from those which obtained during the oolitic period ; 

 but this must have been partly the result of circum- 

 stances, since the specimens found in the newer de- 



* HoXvKTvxoQ (potyptychus), having many folds or wrinkles ; OCOVQ 

 a tooth. 



