114 
INTRODUCTION. 
is that they are more numerous in humid localities 
than elsewhere. For the same reason, they prefer the 
bottom of ravines, and the damp and shady recesses of 
the forest, and choose for their places of shelter strata 
of wet and decaying leaves, and the lower surfaces of 
wood and stones in contact with the soil, or even bury 
themselves in the soil. For though they have the ability 
to retire into their shells and of hermetically closing the 
aperture, yet this power seems to be provided as a de- 
fence against occasional or accidental changes, and not 
against constant or long-continued effects. Hence, they 
are rarely found to occupy positions that are perma- 
nently arid ; and in regions subject to long-continued 
drought, and upon soils which easily give up their mois- 
ture, they are not met with except in low situations, 
near the margins of streams, ponds and swamps. For 
the same reason they are not common on soils which 
become hard and impenetrable on drying, and are want 
ing in sun-burnt wastes and in sandy deserts. The vicin- 
ity of waterfalls, within the influence of the spray and 
mist arising from them, is a favorite locality of inany 
species. Excessive moisture, on the other hand, is very 
prejudicial, for they cannot exist in swamps, or upon 
grounds subject to be overflowed with water, and they 
are destroyed by inundations. The genera, however, and 
even the species, differ considerably in these partic- 
ulars. Glanclina occupies habitually wet and swampy 
grounds, and Suecinea and some species of Pupa are 
often found upon ground in the immediate vicinity of. 
