IOWA ACADEMY OP SCIENCES. 
103 
quite common. Their presence, together with that of the 
‘‘depauperate ” forms, when considered in connection with the 
entire molluscan faunas of the eastern and western parts of the 
state, suggests a climate even drier than that of the eastern 
part of the state, and a surface less abundantly timbered. 
Certainly both modern and fossil faunas unmistakably show'^ 
that the conditions in the eastern and western parts of Iowa, 
during the deposition of the loess were approximately included 
within the bounds of the present extremes presented by these 
regions, and that any attempt to drag into the discussion of 
this subject, conditions either of a glacial climate or of frequent 
and widespread floods and inundations, or of any excess of 
moisture, is gratuitous. 
The conditions which cause the “depauperation” of our 
shells exist more or less all over Iowa to-day, especially west- 
ward, and yet we do not have a glacial climate. If the molluscs 
of the loess be used as an absolute measure of the amount 
of moisture occurring during loess times, then we must conclude 
that Iowa was without streams, for practically no fluviatile 
molluscs occur in the loess, and that there were but few ponds 
in which aquatic molluscs found a favorable habitat, for even 
aquatic Pulmonates are rare in the loess, f the number of ter- 
restrial forms being out of all proportion to that of the aquatic 
forms. 
During the past summer the writer collected several 
thousand specimens in the loess of Mississippi and western 
Iowa, and among them all there were not a half-dozen aquatic 
shells. A list of the modern shells of Iowa shows a large num- 
ber of aquatic species; yet few of these occur in the loess. 
There is also among the modern terrestrial forms a large 
number of those which occur only in very damp places, and 
these, too, are almost wholly missing from the loess. The 
writer is well aware that many of the forms found in the loess 
are often referred to as aquatic or “ semi- aquatic, ” or at least 
as favoring very wet situations. But evidence of this charac- 
ter has been furnished largely by those who are familiar only 
with the molluscan fauna of the eastern part of the country 
where the amount of rainfall is much greater, and where 
surface conditions are not the same as in western Iowa and 
Nebraska; or it has come from so-called “ closet naturalists.” 
♦See also the writer’s paper in Proc. Iowa Acad. Sci., Vol. V, particularly p. 42. 
tPor more detailed comparison see writer’s paper (Ibid.) pp. 43 and 44, and the dis- 
cussion preceding-. 
