IOWA ACADEMY OF 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. Scl., Vol. V, particularly p. 42. 

 tFor more detailed comparison see writer's paper (Ibid.) pp. 43 and 44, and the dis- 

 cussion preceding-. 



