220 



LYELL'S ELEMENTS OF GEOLOGY. 



Fossils of Oolite Group. 



Fiff. 213. 



Belcmnites hastatus. Inferior Oolite. 



Fig. 214. 



Orbicula reflexa, Sow. 

 a. upper valve. 



6. lower or attached valve, and 

 showing part of the upper. 



Fig. 215. 



Ammonites striatulus, Sow, 

 Inferior Oolite and Lias. 



Fiff. 216. 



Terebratula digcna. 

 Inferior Oolite. 



Fig. 217. 



Ostrea Marshii. 

 Middle and Lower Oolite. 



cula (Fig. 212.), Belemnites hastatus {Fig. 213.), and Terebra- 

 tula digona (Fig 216.) 



As illustrations of shells having a great vertical range, I may 

 allude to Trigonia gihbosa (Fig. 205.), which abounds in the 

 Portland stone of Wiltshire, and the Inferior Oolite of York- 

 shire.* Also Ostrea Marshii (Fig. 217.), common to the 

 Cornbrash of Wilts and the Inferior Oolite of Yorkshire ; and, 

 lastly, Orbicula refiexa (Fig. 214.) and Ammonites striatulus 

 (Fig. 215.), fossils common to the Inferior Oolite and Lias. 



Such facts by no means invalidate the general rule, that cer- 

 tain fossils are good chronological tests of geological periods ; 

 but they serve to caution us against attaching too much import- 

 ance to single species, some of which may have a wider, others 

 a more confined vertical range. We have before seen that, in 



'''See Williamson, Proceedings Geol. Soc. No. 47, 



