316 



Fig. 304. 



INTERIOR OOLITE. 

 o 



[Ch. XX. 



Fig. 305. 



Ammonites margaritatus, D'Orb. Syn. A. Stokesii, Sow. 

 Lias. 



Ammonites Braikenridgii, Sow. 

 . Great Oolite, Scarborough. 

 In£ Ool. Dundry; Calvados; &a. 



Fig. 896. 



Fig. 397. 



Ostrea Marshii. J nat. size. 

 Middle and Lower Oolite. 



Ammonites striatulus, Sow. 



i nat. size. 



Inferior Oolite and Lias. 



Such facts by no means invalidate the general rule, that certain fossils 

 are good chronological tests of geological periods ; but they serve to 

 caution us against attaching too much importance to single species, some 

 of which may have a wider, others a more confined vertical range. We 

 have before seen that, in the successive tertiary formations, there are spe- 

 cies common to older and newer groups, yet these groups are distinguish- 

 able from one another by a comparison of the whole assemblage of fossil 

 shells proper to each. 



