146 Call 011 a new Limnceid. 
country upon wliicii the present surface sands-dunes and plains, 
have been deposited. Where that surface lies higher than the 
present level of lake Michigan, that lake has cut into it and 
the bank thus formed has separated the dunes formerly de- 
deposited on this clay from the beach sand which the waves 
are constantly shifting. In some places the old chain of dune« 
seems to have been entirely cut away ; in others, they remain 
fixed and timbered as described. Where the clay surface does 
not rise to the present level of the lake, the beach sand formed 
an uninterrupted slope up the dune, and in such places the 
sand continues to drift, while the waves on the beach continue 
to furnish a new supply. 
When exan,iined, the presence of such drifting dunes will 
probably be found in every instance, to coincide with a depres- 
sion of the clay surface underlying the west Michigan sands 
below the present level of the great lake. 
6th. This clay surface, wherever overspread by sand, a con- 
dition co-extensive with the lower levels of the country to a 
hight estimated at 75 feet above the lake, bears the marks of 
denudation by waves, and has evidently been submerged beneath 
the waters of the great lake. Above that level the character 
of the land is that usually found in drift-clay regions. The 
surface is not generally overspread with sand, the denudation 
which has taken place upon it seems to be the effect of weather- 
ing, and the eastern borders, especially, of such elevated tracts 
are plainly traceable along a continuous level, showing them 
to have been islands at the time when the surrounding plaia 
was submerofed and denuded. 
ON A NEW POST-PLEIOCENE LIMNiEID. 
BY PROFESSOU R. EI-LSWORTH CALL. 
jDcs Moines^ Iowa, 28th November, iSSy. 
In 1S83, while the writer was engaged in the preparation of 
a report on the mollusca of the Great Basin, Pr. R. E. C. 
Stearns kindly submitted for study certain specimens of a fossil 
shell supposed to belong to Pompholyx. The specimens had 
been collected by the California geological survey and were 
