On the Sexual Characters of the Family of Naiades. 119 
inflated ; an arrangement calculated for their accommodation ; as at 
certain seasons, when teeming with ova, they not only occupy all va- 
cant room, in the cavity of the shells, but press the valves apart, so 
as to render them dehiscent at that extremity. 
In the males, there is of course no appearance of ovaries, and 
oviducts, and hence no necessity for an enlargement of the cavity, 
for their accommodation; accordingly, we find their shells more trans- 
verse, and less ventricose ; with the posterior basal, and posterior 
margins, compressed, and the latter more acutely produced. 
This difference in the outlines of the shells of five distinct species 
can be seen, by referring to the annexed diagram. ‘The figures A. 
are female, and B. male shells. 
The evidence of the separate existence of the male sex, isina 
degree negative at least so far as is shown by any examinations of 
the anatomy, that I have been able to make, being founded on the ab- 
sence of ovaries, and oviducts ; but as their presence, or absence, is 
indicated, with certainty, by the form of the shell, is it not reason- 
able to conclude, that this difference in the form of the shell, indi- 
cates, with equal certainty, a distinctness in the sex? 
I have repeatedly tested the correctness of my conclusions with 
the Unio ovatus and nasutus of Say, occidens, sub-ovatus and mul- 
tiradiatus of Lea, rectus of Lamarck, ventricosus and siliquoideus of 
Barnes, with the same satisfactory results, in every instance. 
In my collection are several shells of the U. ochraceus of Say, 
from the Saratoga Lake, and the Schuylkill, the U. alatus of Say, 
from several different rivers west of the Alleghany mountains, and the 
U. xsopus of Green, from the Ohio, and I can distinguish with fa- 
cility by their contours, to which sex they belong, notwithstanding I 
have never exarnined the animals of those species. 
It is equally evident to me, that Barnes’ figures a. and 0. of the U. 
ventricosus, in the 6th. Vol. of the Am. Journal of Science, were drawn 
from female shells; fig. c. from a male, the exterior figure of the U. 
prelongus from a female and the inner from a male; that Lea’s fig- 
ures of the U. occidens and multiradiatus, in the Transactions of the 
American Philosophical society for 1830, and Say’s figure of the U. 
ventricosus, in the fourth No. of the American Conchology, were 
all from female shells. 
It will be found, on pursuing this subject that some which have 
been described, as distinct species, differ from others, only in sex. 
The U. formus of Lea, is probably the male, of the U. triangula- 
