Ch. XX] FOSSILS OF THE PORTLAND STONE. 301 



Fig. 345. 



Fig. 846. 



Isaetroen dblonga, M. Edw. and J. Haime. 



As seen on a polished slab of chert from 



the Portland sand, Tisbury. 



Fig. 347. 



Trigonia gibbosa. a nat. size. 



a, the hinge. 



Portland Stone, Tisbuiy. 



Fig. 34S. 



Cardium dissimile. a nat size. 

 Portland Stone. 



Ostrea expansa. 

 Portland Sand. 



matter may have been, in part at least, derived from the decomposition of 

 vegetables. But as impressions of plants are rare in these shales, which 

 contain ammonites, oysters, and other marine shells, the bitumen may 

 perhaps be of animal origin. 



Among the characteristic fossils may be mentioned Cardium striatu- 

 lurn (fig. 349) and Ostrea deltoidea (fig. 350), the latter found in the 

 Kimmeridge clay throughout England and the north of France, and also 

 in Scotland, near Brora. The Gryphcea virgula (fig. 351), also met with 



Fig. 349. 



Fig. 350. 



Fig. 851. 



Cardium striatvlum. 

 Kimmeridge clay, Hartwell. 



Ostrea deltoidea. Gryphcea virgula. 



Upper Oolite : Kimmeridge clay, i nat. size. 



in the same clay near Oxford, is so abundant in the Upper Oolite of 

 parts of France as to have caused the deposit to be termed " marnes a 

 gryphees virgules." Near Clermont, in Argonne, a few leagues from St. 

 Menehould, where these indurated marls crop out from beneath the Gault, 



