FRESHWATER AND LAND SHELLS. 49 



ferent in the Unto irroratus,* my attention had been particularly ad- 

 dressed to these organs in the few and small species of our vicinity. 



While engaged in this investigation, I received a communication 

 from Dr Kirtland of Poland, Ohio, an ardent and intelligent student 

 of natural history, in which he informed me of his conviction of being 

 able to distinguish the female and male shells of the same species, with- 

 out recourse to the included animal. Yery shortly after this, his con- 

 clusive article on this subject appeared in the American Journal of 

 Science and Arts, Vol. XXVI. It had been a matter of common ob- 

 servation, that individuals of the same species differ very much in the 

 outline of the posterior portion of the shell. In some cases this has 

 been the sole cause of making species, and apparently with reason. 



It has generally been believed by European naturalists, as well as 

 those of our own country, that these animals were androgynous, a prin- 

 ciple so repugnant to nature, that it ought to have excited stronger 

 doubts where the animal structure was so high in the scale of nature, 

 as is the Naiades. 



My attention now became more addressed to sexual characters, and 

 a very short series of examination satisfied me fully as to the establish- 

 ment of the difference of sexes. 



The female sustaining her very large burden, naturally requires 

 more space within the valves — hence we generally find an enlarge- 

 ment of the posterior portion of the shell, differing in its form in vari- 

 ous species. 



In the U. cariosus, ochraceiis, radiatus, luteolus, occidens, purpu- 

 ratus, Sfc, the female is less transverse than the male, being somewhat 

 truncate at the posterior margin, and the greatest diameter is near this 

 portion of the shell. In the species having this distinction, the ovi- 

 ducts will be found to be placed in the posterior portion of the bran- 

 chias. (See PI. XV., fig. 44, 45, representing the oviducts of cariosus 

 and ochraceus.) On reference to the figure of cariosus (Say), in Nich. 

 Ency. (Am, Edit.) art. Conch. PI. III., fig. 2, the female character 

 is evident. The occidens (nobis). Trans. Am. Phil. Soc, Vol. III. 

 PI. X., is female; and purpuratus (Lam.), ater (nobis), Vol. III. PL 



* See Vol. HI., p. 269. 

 VL N 



