Geological Arrangement of British Fossil Shells, 39 



that, in the English formations, this class comprises 22 genera, 

 and .171 species. These abound in our tertiary strata, and 

 are extremely rare in the secondary formations : in fact, only 

 18 species have been detected lower than the plastic clay. 

 They may, therefore, be considered as appertaining to, if not 

 wholly characteristic of, the tertiary formations ; and many of 

 the genera are continued in our present seas. The numerous 

 perforations, both in recent shells, and in those abounding in 

 the beds above the chalk, are the work of these animals. 



In all the other genera of turbinated univalves, the lower 

 margin of the aperture, instead of being either notched or 

 channelled, is entire. Mr. Dillwyn describes the MoUusca of 

 these shells as having jaws, which are formed for feeding upon 

 vegetable substances, and as entirely herbivorous ; the marine 

 genera feeding on ^''Igae, and the fresh-water and land genera 

 on the leaves of vegetables and aquatic plants. These con- 

 stitute the other section of Trachelipodes, called by Lamarck 



Phylliphages, — Of these shells, forming the herbivorous 

 class, 22 genera and 168 species are distributed through the 

 secondary and tertiary formations ; and it will be observed 

 that in the aggregate of genera and species, in each of these 

 great sections, there is a remarkable correspondence. This 

 uniformity of numbers is not maintained so closely in the de- 

 tail, when we arrange the members of these sections according 

 to our three geological divisions. 



Turbinated^ or Turriculated, Univalves, 



ZOOPHAGES, 



or testaceous Mollusca of the carni- 

 vorous class. 

 Species. 

 3 comprised in the lowest, or most 

 ancient division, of strata con- 

 taining organic remains. 

 13 in the secondary formations, or 

 middle division, 

 153 in the tertiary formations, or 

 upper division. 



171 



Genera. 

 2 originated in the lowest division. 

 1 originated in the middle divi- 

 sion. 

 19 originated in the highest division, 

 or tertiary beds. 



22 



Phylli'phages, 

 or testaceous Mollusca of the herbi- 

 vorous class. 

 Species. 

 24 in the lowest division, carboni- 

 ferous formations. 

 65 in the secondary formations 

 (between the mountain lime- 

 stone and the chalk). 

 81 in the highest division (beds 

 above the chalk). 



168 



Genera, 

 12 originated in the most ancient 

 division. 

 5 others added in the middle divi- 

 sion. 

 5 more added in the upper division. 



22 



D 4 



