108 
IOWA ACADEMY OP SCIENCES. 
The presence of this shell-bearing stratum suggests that for 
the period during which it formed the surface soil, and while 
it was slowly accumulating, the conditions in this particular 
locality were more favorable to the growth of land- snails than 
now. There was, probably more vegetation, and hence the 
surface was not so frequently storm-swept as at present. This 
does not necessarily signify that general climatic conditions 
were different, but that these particular banks or bluffs were 
more heavily timbered, with the Missouri river, probably flow- 
ing at its base, its surface conditions being similar to those of 
many timbered bills and knolls between Omaha and Nebraska 
City, west of the Missouri. 
It is interesting to note that between Iowa and Nebraska, 
the Missouri river now flows along the western side of its broad 
valley, and that the adjacent western bluffs are more heavily 
timbered and contain all the living species of molluscs herein 
recorded, with the exception of Nos. 1 and 16, while the more 
remote eastern bluffs are more barren and rugged. The shell- 
bearing band may simply represent the period during which 
the river in its shif tings occupied the eastern part of the valley. 
The foregoing facts lend support to the aeolian theory of the 
origin of the loess, as is shown by the following considerations: 
First — The general manner of distribution of the modern and 
fossil molluscs is essentially the same, this fact indicating that 
they were not carried by waters, but were quietly buried in 
dust. Had they formed a part of river-drifts, they vrould be 
more frequently heaped together, — not scattered as we find 
them in the loess — and fluviatile shells would be more or less 
intermingled. Moreover in many years’ experience in dredg- 
ing in ponds and streams, the writer has seldom seen a land- 
shell which had been carried with the finest sediment into 
ponds or lakes though such shells are sometimes found in sand 
and other coarse material. Currents of water which could 
carry most of the shells now found fossil, would also carry 
coarser material than that which makes up the loess. 
Another fact which bears out this conclusion is the presence 
of opercula in fossil shells of Helicina occulta in the northern 
loess and Helicina orhiculata in the southern loess. As the 
operculum so readily falls from the decaying animal, it would 
scarcely remain in place if the shell had been transported any 
distance. 
Second . — The occurrence of fossiliferous loess chiefly in the 
