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 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 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 
grosvenorii) seem to favor opeu, 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. 
