49 W. Faxon—Dimorphism in the Genus Cambarus. 
Ridgely, S. 60° E.; at Redstone, one and a half miles southeast. 
from New Ulm, S. 25° E.; at Jordan, noted by Foss, Wells & - 
Co., in quarrying and on the site of their mill, S.H. 
Within the till are frequently found layers of sand or gravel, 
which yield the large supplies of water so often struck in dig- 
ging wells. Probably many of these veins of modified drift were 
formed by small sub-glacial streams and therefore cannot be 
regarded as marking divisions of the glacial period, nor even 
any important changes in the overlying ice. It appears, how- 
ever, by shells, remains of vegetation, and trees, found deeply 
uried between glacial deposits in this and adjoining States, 
that the ice age was not one unbroken reign of ice, but that 
this retreated and re-advanced, or was possibly at some time 
nearly all melted upon the northern hemisphere and then accu- 
mulated anew. us periods of ice alternated with interglacial 
epochs, in which animal and vegetable life spread again north- 
ward, following close upon the retreat of the ice-fields. By 
each new advance of the glacial sheet much of the previous sur- 
face would be ploughed up and re-deposited ; hence we find 
only few and scanty remnants of fossiliferous beds in the glacial: 
drift. At the disappearance of the last ice-sheet these drifted 
materials, seldom modified by water in their deposition, formed 
a mantle 100 to 200 feet thick, which throughout the basin of 
oe River almost universally covered the older 
[To be concluded. ] 
Art. VIII.—On the so-called Dimorphism in the Genus 
Cambarus ; by WALTER Faxon. 
THE existence of two forms of the adult male in all the 
species of the genus Cambarus was discovered by Louis Agassiz 
and Henry James Clark. The differences between the two 
forms affect more especially the first pair of abdominal append- 
ages, organs concerned in the act of coition, but also extend to 
the general form and sculpture of the body. In one form (un- 
happily called by Dr. Hagen the ‘‘second form”), the first pair 
of abdominal appendages have a structure nearly like that seen 
in all young males. _ ‘The hooks on the third joint of the third 
(in some species of the third and fourth) pair of legs are small, 
and in the scuipture of the shell and shape of the claws, this 
form approaches the female. In the other form (Hagen’s “first 
form”), the articulation near the base of the first pair of abdom- 
inal appendages is gone and the whole member is much more 
highly specialized, the terminal hooks being horny, more 
widely separated and in every way more highly developed; im 
