406 Fresh-water Shells of Rockbridge County, Va. 
the second species of Unio known to have spines is abundant in 
these rivers. I collected a great variety of them, and observed 
some peculiarities that may not be unworthy of notice. I was 
indebted to muskrats for all the specimens having spines, except 
some very young shells which I took alive ; and thirty or forty oc- 
curred in one spot, in company with the very young of U. lanceo- 
latus. No where else, after a diligent search, could I find the 
young of either species. On the muddy shore of the river where 
muskrats resort, especially about the still water of the dams, I 
procured many specimens of Unio collinus, with spines, which I 
found to vary in the number and position of these spines ; some 
would have a rather long spine on one valve only, others a spine 
on each valve ; others two short ones on the umbo, ranged in the 
direction of the shell’s length ; others with three spines, and some 
with two ranged in a line from beak to base. One specimen has 
a spine just below the umbo, and near the base are two thick 
ones ina line with the shell’s length. In very rocky places where 
the water is rapid I found some living specimens of this species, 
all destitute of spines, which indeed is the condition of a great 
majority of the collinus. It is worthy of observation that this 
spinous character prevails miost among the specimens in still water 
where there is a muddy bottom, and least of all where the water 
is most rapid. On the rocks live vast numbers of Anculosa dila- 
tata, a very variable univalve, and this, with a few specimens of 
Melania virginica, Ancylus rivularis, Planorbis parvus and Palu- 
dina decisa, were the only univalves I noticed. The Unio lan- 
ceolatus is abundant, and may be found alive among the rocks 
and stones. Great numbers always occur, with the animals eaten 
out, about the muskrat haunts and holes in the banks. Unio 
subplanus, Con., is not uncommon in similar situations, and U. 
constrietus, Con., is still more abundant. The latter species and 
U. purpureus of Say, are the two species most commonly ob- 
tained alive. 
It is curious to observe the partiality of the muskrat for par- 
ticular haunts, as is made evident by heaps of shells, the relics of 
their nightly repasts. A particular rock near shore, surroun 
by water, will be seen covered with shells, and on one side of the 
rock a deep stratum of them in the mud, while other rocks neat, 
apparently equally well fitted for the festive board, are never fur- . 
ed with a single shell. I have repeatedly visited one of their 
favorite rocks in a morning, and collected fine specimens which 
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