Ch. XX.] FOSSILS OF THE POETLAND STONE. 301 



Fig. 8'i5. 



Fig. 846. 



Isastrona dblonga, M. Edw. and J. Haimc. 



As seen on a polished slab of chert from 



the Portland sand, Tisburj, 



Trigonia gibhosa. i nat. size. 



o, the hinge. 



Portland Stone,"Tisbury. 



Fig. 847. 



Fig. 348. 



Cardium dissimile. i 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- 

 lum (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 Gryphma virgula (fig. 351), also met with 



Fig. 349. 



Fig. 850. 



Cardium stnatulum. 

 Kimmeridge claj'-, Hartwell. 



Fig. SSL 



Ostrea deltoidea. Oryphma virgula. 



Upper Oolite : Kimmeridge clay. -\ 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 maris crop out from beneath the Gault, 



