54 DISTRIBUTION IN TIME AND SPACE. 



fulness according to the correctness and extent of our knowledge 

 of the data from which they are derived. 



In the same manner, the geological history of each genus and 

 species being carefully noted, we postpone to a concluding 

 volume certain geological considerations which properly include 

 the whole of .the mollusca, rather than particular groups. 



It may not be out of place to recall the observation of Dillwyn 

 that the shells of carnivorous gasteropods, almost wanting in 

 palaeozoic and secondary periods were then replaced in the 

 economy of nature by the almost extinct order of tetratranchiate 

 cephalopods.* 



Bronn has prepared the following synoptical table of the num- 

 ber of genera and species of prosobranchiates occurring in each 

 formation; aggregating T123 species: it would be largely in- 

 creased, but its relative proportions probably not much changed, 

 by the incorporation of material since made known to science. 



/Silurian, 164 species, 11 Genera. 



"VsTsneoii J Devonian, 244 20 



737 species . 4 Carboniferous 313 

 57 genera. ( Permian> 17 



SECONDARY. i Triassic, 393 



^764 species. < Jurassic, 488 



166 genera. (Cretaceous, 883 



v-kmi-A-r-rr I /^ * ) O 



26 



36 " 



1764 secies. J Jurassic 488 56 " 



74 " 



TERTIARY 4622 " 179 " 



The relations of the tertiary with the recent mollusca are daily 

 appearing to be more intimate. It is probable that a very 

 considerable proportion of its species will be found to be sy- 

 nonymous with existing forms, and that the juster and more com- 

 prehensive views of nature which have now obtained (and which 

 are the happy result of the development theory whatever may be 

 said of the truth of its ultimate conclusions), will enable us to 

 make proper allowances for influences producing variation in 

 fossils as we do in recent species. Time has not been the only 

 factor : as many of our so-called extinct species are obtained 

 from particular local deposits, their characteristics are frequently 

 more local and varietal than specific. Hundreds of cases might 

 be cited of variations from a known specific type of recent mol- 



* This work, vol. i, p. 48, where is also a table of the number of genera 

 of mollusca belonging to each geological period. 



