244 
IOWA ACADEMY OF SCIENCE 
as the shells were gradually buried, the fossils retained the same 
peculiarities of distribution. In localities where more than one 
loess occurs, changes in local conditions frequently produced one 
loess with fossils, and another without. 
Creeping and slipping evidently do not materially influence the 
peculiarities of distribution of the fossils, for these are related to, 
and consistent with those of the living fauna. 
Equally unfortunate is Professor Todd’s attempt to explain the 
absence of the fresh-water mollusks from the loess by reference to 
the possible coldness of the water, and to its muddiness. 
Modern fresh-water shells are exceedingly abundant in some 
localities in the far north ; on the other hand they are almost com- 
pletely absent from the loess of the far south, where it is certain 
that at no time in the later history of the continent, cold could have 
caused this absence, and where the remains of land shells are in 
large part those of species, which are almost sub-tropical. Through- 
out the Mississippi valley modern fresh-water shells are abundant, 
and they were probably equally abundant during the time of the 
deposition of loess, for the much more abundant terrestrial shells 
point to conditions similar to those of the present. The fresh- 
water shells were not buried in the loess simply because it was not 
deposited in water, in which they lived. 
The muddiness of the streams equally fails to account for the 
absence of fresh-water mollusks. Even continually muddy water 
is not necessarily fatal to mollusks. For example in the brick-red 
Washita river of Oklahoma, with its perpetual burden of silt which 
exceeds in amount for every cubic unit even that of the Missouri, 
fresh-water mollusks are locally common. 
The absence of mollusks from a large part of the Missouri river 
is due probably in very large part to the constant rapid shifting 
of the bottom of that stream. But even granting that mud destroys 
molluscan life, the absence of fresh-water mollusks from the Mis- 
souri river loess is not explained on this ground. This loess ex- 
tends along many of the tributaries of the Missouri, and these 
tributaries are not perpetually muddy, and nothing indicates that 
they ever were muddy throughout the year, like the Missouri. 
These tributaries now contain an abundant fresh-water molluscan 
fauna, and they have no doubt sustained it during all the time 
which has been favorable to the terrestrial mollusks which have 
inhabited the region from the time of the beginning of the loess 
deposition. Yet no fluviatile fossils have been found in the loess 
bordering them. 
