100 PROSOBRANCHIATA. 



branchiates — the marine zoophaga, whilst the rounded aperture 

 and its efficient operculum belong to the phytophagous groups. 

 In going over the genera of marine prosobranchiates another 

 general law appears to coexist with the foregoing divisions, 

 namely, that the zoophaga are the most active, and are frequently 

 deep-sea animals, whilst the phytophaga are necessarily more 

 confined to shallow water, between tides, etc., where their food 

 is more readily obtainable. Some of the zoophaga prefer a rocky 

 station, whilst others affect sandy or muddy bottoms ; the little 

 genus Stylifer is parasitic upon echini, etc., immersed in which 

 it dwells, and some other genera habitually seek special stations, 

 as Pedicularia and Magilus upon corals, certain Vermetidse 

 upon other shells, etc. On the other hand, numerous animals 

 dwell vipon and within the substance of the shells of univalve 

 mollusca, including sponges, worms, corals, molluscoids, etc., 

 not to mention many of the true mollusca, and especially bivalve 

 species. 



Bronn has prepared the following sjnioptical table of the 

 number of genera and species of prosobranchiates occurring in 

 each geological formation; aggregating tl23 species: it would 

 be largely increased, but its relative proportions probably not 

 much changed, by the incorporation of material since made 

 known to science. 



T^ r Silurian, 



Paleozoic. -rw • ' 



^,„^, . Devonian, 



U1 species, j Carboniferous, 312 

 57 genera. \ r, • ih 



" [ Fermian, 



Secondary. TTriassic, 



1164 species. ■< Jurassic, 

 166 genera. (Cretaceous, 



Tertiary, 



164 



species. 



11 



genera. 



244 



u 



20 





u 



312 



u 



26 





a 



n 



u 









393 



a 



36 





u 



488 



u 



56 





u 



883 



u 



74 





a 



4622 



u 



179 





u 



The relations of the tertiary with the recent mollusca are daily 

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

 siderable proportion of its species will be found to be synony- 

 mous with existing forms, and that the more comprehensive 

 views of nature which have now obtained (and which are the 

 happy result of the development theory — whatever may be said 

 of the justness 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, probably, 

 are frequently more local and varietal than specific. Hundreds 

 of cases might be cited of variations from a known specific type 



