290 



SHELLS AND SHELL-FISH. 



PART I. 



Analogies of the Genera of the ALASMODONTiNiE. 



Genera. 



Uniopsis. 



Analogical 

 Characters. 



'Inequilateral •,'\ 

 the cardinal I 

 teeth reced- ! 

 ing from the [ 



Sub-families 



of the 



Unionidcc. 



Onioning. 



Genera 



of 



UnionincB. 



Unio. 



1 anterior J 



L margin. J 



rCuneate, andT 

 Alasmodon. < nearly equi- >■ Hyrin^. ^glia. 



C lateral. 3 



Calceola. ? Alasmodonin*;. Potomida. 



r Winged ; the") 

 ^ , .1 bosses small I 



Complanana. -J ^^^j ^^^^^ 



(. compressed. 3 

 r Oblong oval,T 

 Margaritana. \ greatly S-Iridin^. Mysca, 



C lengthened. J 



Sub-genera 



of 



Unio. 



Unto. 



Cunicula. 

 Megadomus. 



ANODONTiNiE. Lymnadia. Tfieliderma 



Ligumia. 



(271.) Having brought our rapid exposition of this 

 family to a close, we may now offer a few general re- 

 marks upon the whole. The UnionidcE, of all the groups 

 of the DiTHYRAj is that which contains the gi*eatest 

 modifications of form ; insomuch that it becomes 

 almost impossible, in some of its minor divisions, to 

 determine which are species, and which varieties. 

 Hence has originated the idea, entertained by several 

 conchologists, that they constitute but a single genus ; 

 while others have gone so far as to suspect that there 

 is in reality but one species of Unio, and '' perhaps of 

 the whole family : " such, at least, were the sentiments 

 entertained some years ago, although we believe they 

 no longer exist. Upon this principle, the equally ex- 

 tensive and varied family of Helicidce should form but 

 one genus, or rather but one species, which might be 

 called Helix or Limax terrestris, because it lived upon 

 land.* It is useless to prove, that by this false and 

 vicious sort of generalisation, we should return back to 



* The naked slugs and the land snails pass into each other by such gra- 

 duated shades, that no " well-marked and absolute characters" can be 

 found to separate them. 



