

On the Sexual Characters of the Family of Naiades. 119 



inflated j 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, is in a 

 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- 

 tiradiatusof 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. zesopus of Green, from the Ohio, and I can distinguish with fa- 

 cility by their contours, to which sex they belong, notwithstanding I 

 have never examined the animals of those species. 



It is equally evident to me, that Barnes' figures a. and b. 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. 

 praelongus 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- 



