40 Geological Arrangement of British Fossil Shells, 



There remain about 62 species of fossil univalves which are 

 not turbinated. 



Hence it will be perceived that the turbinated univalves 

 of the older strata or rocks belong almost entirely to the 

 herbivorous family, 12 genera having originated there, which 

 have been perpetuated through all the successive strata, 

 and still inhabit our waters ; that in the middle series of 

 formations, this preponderance of animals possessing similar 

 habits was preserved ; and that, in the last series, after the 

 chalk was deposited, this order was suddenly reversed, in the 

 proportion of 5 to ] 9. 



Mr. Dillwyn observed that all the marine Trachelipodes, of 

 the herbivorous tribes, in the ancient strata, are furnished with 

 an operculum, seemingly intended as a protection against the 

 Cephalopodes, or carnivorous order of NauUYi, Ammonites, &c., 

 which, at that time, abounded in the seas. After the epoch 

 of the extinction of this order (which terminated chiefly with 

 the chalk), numerous unoperculated genera appear, as if no 

 longer requiring such a shield to protect them from an extinct 

 enemy. As carnivorous turbinated univalves were, almost en- 

 tirely absent from the strata w^hich contained the Ammonites, 

 the iVautilidiae, and the Belemnites, so the extinction of these 

 immensely numerous tribes, being also carnivorous, or preda- 

 ceous, was counterbalanced by the creation of a multitude of 

 new genera, possessed of similar appetences. 



Recurring again to our table for illustration of these positions, 

 we observe that only 3 genera and 18 species of carnivorous 

 turbinated univalves were coeval with the Cephalopodes, com- 

 prising 200 species, in the secondary formations ; but that the 

 same strata contained 17 gen. and 87 species of Phylliphages. 



When the Cephalopodes ceased with the chalk, at the 

 same time with the numerous families of fossil ^chinidiae, the 

 Trigoniae, and nearly all the Terrebratulae, they were re- 

 placed by 19 genera and 153 new" species of Zoophages. 



As we have the materials before us, and as the enquiry is 

 not devoid of interest, it may be worth while to compare the 

 existing classes of shells with corresponding series in the an- 

 tediluvian creation. 



Testaceous Mollusca of the pre- 

 sent world, ascertained from the 

 Index Testaceologicus of Mr. 

 Wood, last edition 



Species of British fossil shells, 

 heretofore described, dispersed 

 throughout the entire range of 

 the formations 



Total 



Species. 



2895 

 1265 



