100 IOWA ACADEMY OP SCIENCES. 
prairie ponds, but as a rule, rough, rolling timber-areas 
are favored. Here an abundance of food (for nearly all are 
herbivorous) and more or less shade and protection are fur- 
nished by the vegatation. As we recede from the timber- 
bordered streams, the number of species and specimens grows 
less, and the writer knows, from personal experience obtained 
in various parts of the state, that large prairie-areas of that 
character may be searched in vain for any trace of a land- 
mollusc. In the eastern part of the state, with its more rolling, 
timber-covered surface, almost every locality — certainly every 
county — presents numerous favorable locations for colonies 
of snails, but as the collector crosses the state westward he 
finds that, in species and in specimens, the molluscan fauna 
grows poorer, the timber-fringed streams, or ponds and lakes, 
alone marking the favorable localities. 
If careful observations are made even in the best of these 
collecting-grounds, whether in the eastern or western parts of 
the state, it will be found that much variation and inequality in 
local distribution exists. One hillside may present certain 
species, while the next, perhaps across a narrow ravine, will 
show a wholly different series, and a third near by may have 
none at all. A species which in one spot is the prevailing type, 
may, only a few rods, or even feet away, be wholly, or in part, 
supplanted by another. This is sometimes due to differences 
in the abundance of trees and vegetation furnishing food, and 
to other variations in the character of the surface, but often it 
seems to be a mere accident. 
The number of individuals of any, or all, species in a given 
locality is also very variable. In the most favorable spots, 
however, especially on higher grounds, one seldom finds many 
individuals together. Even such species as Zonitoides arboreus, 
Z. minusGulus, Vitrea hammonis, Gocliliopa lubrica, Succinea obliqua, 
S. avara, etc., which may often be found in large numbers 
under leaves or sticks and logs in comparatively low places, 
usually show fewer and more scattered specimens on hillsides, 
etc., especially in more open places To get a good set of any 
species in such localities, the collector must work over a con- 
siderable area, but in doing so, he will almost invariably find 
individuals of several species mingled promiscuously. If he 
compares the molluscan faunas of the eastern and western parts 
of the state, he will find that as stated, the number of species 
and individuals in the eastern part is, as a rule, greater. He 
