DISTRIBUTION IN TIME AND SPACE. 53 



ties of habit and of distribution characteristic of particuhir groups 

 will And place in the Aolumes wherein they will be described. 



Whilst the prosobranchiates are typically marine animals, 

 there are many exceptions to the rule ; for not only do we find 

 a certain number of genera inhal)iting brackish water, but some 

 live in fresh water only ; and others again, are terrestrial. It is 

 not without modification of the breathing organs that such diver- 

 sity of station exists, and this modification is co-existaut with 

 otlier adaptations. 



Whilst the pulmonifcrous moUusks have no operculum, the 

 terrestrial and fluviatile sections of the prosobranchiates are pro- 

 vided with a very efficient one, completely closing the aperture 

 of the shell. The canaliculate aperture of the shell, the opercu- 

 lum too small to fill its aperture, and, frequently, the want of an 

 operculum are characteristic of the major portion of the proso- 

 branchiates — the marine zoophaga, whilst the rounded aperture, 

 and its efficient operculum belong to the phytophagous mollusks. 

 In going over the groups of marine prosobranchiates another 

 general law appears to coexist with the foregoing divisions, 

 namely, that the zoophaga are the most active, and are deep sea 

 animals, whilst the phytophaga are necessarily more confined to 

 shallow water, between tides, etc., where their foodris more 

 readily obtainable. Some of the zoophaga prefer a rocky station, 

 whilst others affect sandy or muddy bottoms ; the little genus 

 Stjdifer 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 Yermetidw upon 

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

 upon and within the substance of the shells of univalve moUusca, 

 including sponges, worms, corals, moUuscoids, etc., not to men- 

 tion many of the true mollusca, and especially bivalve species. 



The influence of structure and habit, of temperature, the chemi- 

 cal composition of the sea-water, etc., upon the actual geographi- 

 cal and bathymetrical distribution of the species, and the pre- 

 sentation of the resultant faunal regions, as already stated must 

 be reserved for a future occasion. I purpose, in describing the 

 species to note all facts of this nature that have been heretofore 

 recorded, as a contribution towards generalizations, which, 

 already become interesting and important, will increase in use- 



