100 IOWA ACADEMY OF 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 

 growls poorer, the timber-fringed streams, or ponds and lakes, 

 alone marking the favorable localities. 



If careful observations ara 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. iiiinusculKs, Vitrea hcuiunonis, Cochliopa lubrica, Succinea oblujua, 

 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 



