120 Correspondence. 
found only in the northeastern portion of the state. The first of these 
two is a minute yet hardy species, circumpolar in its geographic range 
and possessing a high antiquity geologically. The other was, until 
very recently, known only from fossils. A few years ago it was dis- 
covered in considerable numbers living near Iowa City and later in 
Hardin county, Iowa. The local distribution of the living forms of 
this species is very remarkable, and in both of the places mentioned 
it is extremely limited, being scarcely an acre in extent. It thus ap- 
pears that the once abundant and widely distributed species is now on 
the verge of extinction. 
Attention has lately ^ been called to a striking difference in the fac- 
ies of the living Unionidfe of central and eastern Iowa, and incidentally 
to some peculiarities in the distribution of the gasteropods over the 
same region. Extensive collecting among recent mollusca in various 
parts of the state revealed that certain species, especially land forms, 
were less abundant in the western and central, than in the eastern, 
portions of Iowa ; and that some of these forms were much more 
plentiful towards the northern than towards the southern, limits of the 
latter district. The occurrence in great abundance, in the loess over 
central Iowa, of certain gasteropods which are now extinct throughout 
the tract, and the recent discovery of some of these same forms living 
in great numbers in the northeastern part of the state, suggested that 
a part at least of the present molluscan fauna of Iowa was derived from 
the northeastward ; and that the migrations of the mollusks probably 
began immediately upon the recession of the great ice sheet. The ex- 
istence of the extensive "driftless " area of southwestern Wisconsin — 
once an immense island in the great mer de glace — afforded considera- 
ble plausibility for the inference, but in the absence of suitable mater- 
ial from this region the actual presence of various living mollusca 
could not be satisfactorily determined. A short time ago, however, 
some of this material was supplied from southeastern Minnesota. 
Among the living forms represented were Helicina occulta Say, and 
Vallonia pulchella Miiller, both from within the limits of the "driftless" 
tract. The discovery is of great interest as corroborating the supposi- 
tion already set forth, and it is not unlikely that the first of these two 
species will be found in other localities in northeastern Iowa and the 
adjoining portions of the contiguous states ; its peculiar and strictly lo- 
cal distribution will, however, tend to render detection quite diflficult. 
The southern limit of the vast ice sheet of the principal glacial epoch 
has been made out with an eminent degree of accuracy, and over the 
Mississippi valley coincides approximately with the Ohio and Missouri 
rivers. As the ice melted along the front of the retreating glacier, 
enormous accumulations of glacial silt and debris were deposited. But 
whether the deposition of the loess was contemporaneous with the 
closing stage of the earlier glacial epoch, as advanced byChamberlin 
and Salisbury ;- or was during the episode immediately succeeding the 
^ Keyes : Annotated Cat. Mollusca Iowa, Bui. Essex Inst., vol. xx. 
■■'6th Ann. Rept. U. S. Geol. Sur., p. 305. 
