IOWA ACADEMY OP SCIENCES. 99 



Fossils are more abundant in the vicinity of streams because 

 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 to-day. 



It has already been pointed out by the writer* that the loess - 

 fauna of any section of the country closely resembles the mod- 

 ern moUuscan fauna of the same section, the characteristic 

 fossil species being, for the most part, characteristic species of 

 the modera 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 connection 

 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, land-shells form the 

 characteristic fauna of the loess, and with two or three 

 exceptions the same species may be found living within the 

 borders of our state to-day. 



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 vegetation determines largely the distribu- 

 tion of the molluscs is shown by the fact that timber or brush- 

 covered areas, remote from streams, are quite likely to yield 

 plenty of shells. A few species (as for example Succinea 

 grosvenoi'ii) seem to favor open, rather grassy places, and a few 

 others may be found among the weeds and bushes skirting 



Proc. Iowa Acad, of Sci., Vol. V, pp. 33-41. 



