Ch. XX.] 



FOSSILS OF THE PORTLAXD STOXE. 



395 



ruinous matter may have been, in part at least, derived from the de- 

 composition of vegetables. But as impressions of plants are rare in 



Fisr. 3 



. J nai, size 

 Portland Stone. 



Fie. 354. 



Ostrea expansa. 



Portland Sand. 



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 Carclium stria- 

 tulum (rig. 385) and Ostrea deltoklea (fig. 386), the latter found in 

 the Kimmeridge clay throughout England and the north of France, 

 and also in Scotland, near Brora. The Gryphcea virgula (tig. 387), 



Fiff. 355. 



Fig. 3S6. 



Fisr. 3S7. 



Cardium striatulum. 

 Kininieridsre clar, Hart-well. 



Ostrea deltoklea. 

 Kimmeridge clay. -J nat. size. 



GrypTicca (Fxogyra) vir- 

 gula. Kimmeridse clav. 



Fig. 3SS. 



also met with 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 Aro-onne, 

 a few leagues from St. Menehould, where these indurated marls crop 

 out from beneath the gault, I have seen them, on decomposing, leave 

 the surface of every ploughed field literally strewed over with this fos- 

 sil oyster. The Trigonellites latus (Aptychus, of 

 some authors) (fig. 388) is also widely dispersed 

 through this clay. The real nature of the shell, of 

 which there are many species in oolitic rocks, is 

 still a matter of conjecture. Some are of opinion 

 that the two plates formed the gizzard of a 

 cephalopod; for the living Nautilus has a gizzard 

 with horny folds, and the Bulla is well known to 

 possess one formed of calcareous plates. 



The celebrated lithographic stone of Solenhofen, in Bavaria, be- 

 longs to one of the upper divisions of the oolite, and affords a re- 



Trigonellites latus. 

 Fvknmeridge clay. 



