Remarks on the Unios of the United States. 43 
names. — Lamark has made three or four species in this 
manner of the U. fluviatilis. 
There is also a striking resemblance between some of the 
Unios of the Atlantic States and some species found in onx 
Western waters; thus the U. nassutus of the Delaware, 
approximates so nearly the U. rectus Lam., often called 
U. prcelongus, of the Ohio, that on comparing a number, 
we have been almost led to the conclusion that they are 
merely varieties of the same species, produced by difference 
of food and locality — they are certainly as much like each 
other as the U. fluviatilis of the two rivers. That one 
should be a much thicker and larger shell than the other 
is no objection; for we find that those specimens of the 
Unio taken from deep water, are generally Jjiin and fra- 
gile, while individuals of the same species found in shallow 
and sluggish streams, are much more ponderous — Again, 
the U. carinifera of Lamark, which is a true Fluviatilis^ 
very much resembles some specimens of the U. alatus in 
its wing like appendage, though there can be no doubt that 
this last shell belongs to a distinct species, if not to another 
Genus. If we were authorized to adopt the theory that 
some of the described species of Unio generated promis- 
cuously together, most of our present difficulties might be 
very readily removed. Certain varieties in the contour of 
many species we know take place in consequence of a pre- 
ternatural enlargement of the shell in certain directions, 
owing probably to a disease of the animal, which has pro- 
duced an elongation of his mantle in that direction. The 
amount of this monstrous growth can be readily determined 
by the configuration of the impression made by the mantle 
of the animal on the surface of the nacre, near the margiii 
of the shell. Besides the above regular but preternatural 
formation, many singular distortions of the Unio frequent- 
ly occur. Not to mention others, in my cabinet there is a 
