316 



Fig. 394. 



IKFERIOR OOLITE. 



[Ch. XX 



Fig. 395. 



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

 Lias. Great Oolite, Scarborough. 



Inf. Ool. Dundry; Calvados; &c. 



Fig. 896. 



Fig. 39T. 



Ostrea IfarsJiii. | 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 "v^hole assemblage of fossil 

 shells proper to each. 



