THE DISTRIBUTION OF LOESS FOSSILS 1 23 



the same species thrive, and in all probability did thrive in the 

 past, in just such situations. 



Manifestly, if we would judge of the conditions under which 

 the fossils existed and were finally buried in the past, we must 

 understand the conditions under which the same species exist 

 today. It has already been pointed out by the writer 1 that the 

 loess fauna of any section of the country closely resembles the 

 modern molluscan fauna of the same section, the characteristic 

 fossil species being for the most part characteristic species of the 

 modern fauna. During the past summer the writer made more 

 extended studies of fossils in widely separated loess regions, 

 notably in Mississippi, Iowa (both eastern and western), and 

 Nebraska, which strongly emphasize the foregoing fact. As 

 questions of general geographical, as well as local, distribution 

 of fossil and modern molluscs are of great importance in con- 

 nection with any attempt at an explanation of the manner in 

 which loess was deposited, the following remarks are offered as 

 preliminary to further detailed reports upon the distribution of 

 the loess species and of their modern representatives. 



In Iowa and Nebraska, as elsewhere, the land shells form the 

 characteristic fauna of the loess, and with two or three excep- 

 tions the same species may be found living within the borders of 

 our state today. 



The student who goes to the field to study the living forms 

 in their natural environment, if his studies be sufficiently 

 extended, will be struck by the many seeming eccentricities in 

 distribution. He will, however, observe that our land molluscs 

 as a rule favor the regions adjacent to streams — especially the 

 rough, rugged hills which so often border them. This fact, 

 however, seems to be dependent upon another, equally interesting 

 and long well known — namely, that our timber areas for the 

 most part skirt the streams — and that this distribution of vege- 

 tation determines largely the distribution of the molluscs is 

 shown by the fact that the timber or brush-covered areas remote 

 from streams are quite likely to yield plenty of shells. A few 



' Proc. Iowa Acad, of Sciences, Vol. V, pp. 33, 41. 



