. 
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, ete., 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 i in which 
it dwells, and some other genera habitually seek special stations, 
as Pedicularia and Magilus upon corals, certain Vermetide 
upon other shells, ete. On the other hand, numerous animals 
dwell upon and within the substance of the shells of univalve 
mollusea, including sponges, worms, corals, molluscoids, etc., 
not to mention many of the true mollusea, and especially bivalve 
species. 
Bronn has prepared the following synoptical table of the 
number of genera and species of prosobranchiates occurring in 
each geological formation; aggregating 7123 species: it would 
be largely increased, but its relative proportions probably not 
much changed, by the incorporation of material since made 
known to science. 
Piero | f Silurian, 164 Species, 11 Beutss 
Ha" species Devonian, 244 20 
57 ee. | Carboniferous, 312 i 26> Fe 
2 * | Permian, 17 i 
SECONDARY. (oe 393 a0 = 
1764 species. < Jurassic, 488 a 56 
166 genera. Cretaceous, 883 C 1) 
TR BRIDARNG hs. eis Ae ape 4622 fe Wii oe 
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 
