38 Geological Arrangement of British Fossil Shells, 



Second and Third Divisions. 

 First Division, maining strata, above tlie 



Ancient strata, including lias. lias, up to diluvium. : 



Species 36 Simple univalves - 565 species. 



67 Simple bivalves - 516 



J 51 Complicated bivalves ^ i 147 



^83 Multilocular univalves 1473 



237 1028 



It will thus be perceived that the number of complex spe- 

 cies in the first division is nearly equal to those in the im- 

 mense series of succeeding strata, 134 being peculiar to the 

 lowest, and 147 to the remainder. But the individuals are 

 infinitely more numerous in the older strata than in the later, 

 and give a more decided character to those formations than 

 appears from a comparison of genera or species ; and the class 

 of complicated bivalves is wholly limited to this older division. 

 The difference is yet more striking when we compare the first 

 with the third division; the simple univalves in the former 

 being to those in the latter in the proportion of 1 to 7 ; but the 

 complicated species, in the same divisions, are in the reverse 

 ratio nearly of 17 to 1. 



On comparing the proportions which the classes of shells 

 under each division bear to each other, differences equally 

 remarkable are observable. Thus the univalves in the first 

 division are to the complex species as 1 to 4 ; in the second, as 

 1 to Ij only; and, in the third, as 32 to 1. 



In concluding this summary, we may repeat, as a general 

 rule, that the ancient formations are characterised by compli- 

 cated shells, the middle series by bivalves, and the upper by 

 simple univalves. 



In illustration of the habits of two great classes of testaceous 

 Mollusca, the investigation of Mr. Dillwyn developes some 

 unexpected differences in their modes of existence; and the sub- 

 ject is the more interesting, as being singularly confirmatory 

 of other essential changes observed in the structure of ante- 

 diluvian animals, at different epochs or stages of the creation. 



This examination is limited to the turricuiated univalves, of 

 which our list contains 337 species. 



All those Mollusca whose shells have a notch or canal at 

 the base of their apertures, are furnished with the pov/er of 

 perforating shells, and other hard substances, by means of a 

 retractile proboscis. In Lamarck's arrangement of invertebral 

 animals they form a section of the Trachelipodes, under the 

 name of 



ZoSphages. — On comparing Lamarck's list of carnivorous 

 or predaceous genera with our present Table, it will appear 



