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 differentes 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, Hyalea, Cleodora, Cuvieria, Cymbulia, 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) 
