PRESENT HISTORY AND DISTRIBUTION 61 



scattered bush has its complement of Helix desertorum 

 (Plate XIII., Fig. 34), while the ground beneath is 

 strewn with the dead shells of past generations. 



All the terrestrial Snails are chiefly crepuscular or 

 nocturnal in their habits, but will issue forth on 

 cloudy or wet days. 



While the haunts of the Mollusca are similar in 

 all parts of the globe, the distribution of the various 

 members over the world's surface differs, just as it 

 does in other divisions of the animal kingdom, on 

 account of the differences of environment. 



Obviously the geographical distribution of the 

 Mollusca falls primarily into " marine " and " non- 

 marine." 



Dealing with the marine first, it is fouud that the 

 pelagic forms are the more widely distributed, but 

 may nevertheless be described as belonging to polar 

 and tropical provinces. The North and South Polar 

 forms differ : Clione limacina and Limacina helicina 

 belong to the former ; while among the latter are 

 Limacina antarctica and Spongiobranchia australis. 

 Argonauta, Hyalcea, Cleodora, Cuvieria, Cynibulia, etc., 

 frequent the warmer waters. 



The abyssal fauna, living under uniform condi- 

 tions, is widely distributed, and cannot be broken 

 up into provinces, since identical species will occur 

 either in northern and southern parts of the same 

 ocean or in several different oceans. 



A few of the shore Mollusca are very widely dis- 

 tributed, such as the Common Mussel (Mytilus edulis) 



