OF THE FAMILY OF NAIADES. 407 



be scarcely more unlike than the smooth and radiated sili- 

 quoideus of Barnes and the beautiful tuberculated lacrymo- 

 sus (nobis); and the same remark may be applied to the 

 cylindrlcus and ulatus of that excellent conchologist Mr Say. 

 Many other species could be thus contrasted, but I deem the 

 above sufficient, upon examination, to prove the justness of 

 my remarks, and the necessity, in the present state of our 

 know^ledge, to retain the species, whatever may be the changes 

 in the genera*. 



In a preceding paper on the Uniones I said something on 

 the habits of the animal. I wish now to mention the simple 

 fact that I have kept several specimens about ten months in 

 a basin changing the water every five or six days. During 

 this period they passed through the winter without any 

 change in their usual habits, and nothing in the shape of food 

 was given during the whole period. 



This truly interesting family presents us with very diffi- 

 cult specific characteristics, rendered so by the species con- 

 stantly approaching in similitude to each other, and by the 

 change made in them by age, locality, and exposure. 



I propose to offer a few observations on the principal cha- 

 racters, in which it will be seen how little we can depend on 

 any one of them, and shall begin first with the teeth. 



Teeth. In the species of the Unio these have been used as 



* In a letter addressed to me by William Cooper, Esq., an intelligent natu- 

 ralist of New York, he says, " There are now, I think, not less than thirty North 

 American species of Unio well established, and perhaps seven or eight more. 

 That they are species, each perpetuating its peculiar form, subject to certain va- 

 riations, but permanent within fixed limits, seems tome the most rational opinion, 

 although some of our most judicious naturalists think otherwise. Your account 

 of the animal of the U.irroratus -nSovA^ a strong argument in favour of this belief, 

 for it proves that to be beyond doubt as distinct a species as any in any class of 

 animals. Yet this may always be known with certainty by the shell, which, 

 though so well characterised, is not, however, more different from the rest of the 

 genus, than they are from each other, and frequently still less so. If, therefore, 

 this difference is found to be constantly indicative of a species in one instance, it 

 must also be in others. I believe that our lakes and rivers contained the same 

 form of shells at the creation and ever since that they do at this day. If they are 

 hermaphrodite per se, as is said of them, it could not be otherwise ; and if the 

 contrary were admitted, natural history would not deserve the name of a science." 

 VOL. III. 5 L 



