IOWA ACADEMY OP SCIENCES. 
101 
will also find that there are certain rather striking differences 
between sets of some of the species taken at opposite extremi- 
ties of the state. Those from the eastern part are likely to 
average larger in size and to be thinner-shelled, resembling 
more nearly representatives from the eastern part of the 
country, while the western forms are smaller and heavier. 
This is especially true of Polygyra multilineata, Zonitoicles minus- 
culus, SuGGinea obliqua, S. avara, and other species of the kind 
which are sometimes found in rather low places, but which also 
occur on higher grounds, especially westward. This is prob- 
ably due chiefly to the scarcity of forests in the western and 
central parts of the state, where the rather scant groves usually 
consist of scattered and stunted trees, being quite different 
from the more vigorous forests of the eastern part. That this 
view is correct, is further attested by the fact that the same 
species of molluscs, when occurring on comparatively barren 
or nearly treeless areas in the eastern part of the state, usually 
show the characters of the western types — namely, the smaller 
size and sometimes heavier, or at least more compact, shell. 
If the student will study the molluscs of a given region for 
a number of years, he will find that from year to year the 
abundance of the several species varies, some even running 
out entirely, while others unexpectedly appear. The writer 
has watched a number of localities near Iowa City for many 
years, and has found this variation often striking. 
If, now, the distribution of the fossils in our loess is com- 
pared with that of the modern shells, a remarkable similarity 
is evident. The best collecting grounds are near streams, 
while the clay of the remote prairie is usually barren. Where 
fossils are abundant, one exposure contains species of one kind, 
another near by presents a new, or at least a different list, while 
still another has none, and the same variation which may be 
observed in the local distribution of the recent shells in any 
restricted locality, will be exhibited in individual exposures of 
fossiliferous loess. 
In horizontal distribution the fossils show the same mode of 
distribution as that already noted in the modern forms. The 
specimens are not heaped together, but are scattered about 
like the modern shells, usually a number of species mingled 
together, but in unmodified loess invariably not crowded, so far 
as the writer’s experience goes. 
The vertical distribution of the fossils also conforms to the 
