8 PHYSAD.E. 



might attach themselves. That the density of 

 the internal and external air is sometimes dif- 

 ferent in this family, and in the terrestrial 

 Pneumobranchiata, is proved by the slight 

 clicking sound that frequently accompanies the 

 opening of the branchial foramen. 



The head varies somewhat in shape, but is 

 never decidedly proboscidiform as in the more 

 highly organized Melaniadse. I have never 

 been able to discover that the Limneans or 

 Helices possess true organs of vision; but it is 

 probable that they can distinguish light from 

 darkness. In the other freshwater families de- 

 scribed in this work, the power of vision, or 

 sensitiveness to the action of light, is rendered 

 evident by intercepting it with an opaque ob- 

 ject, when they instantly retract; and I have 

 even observed the protruded branchial canal of 

 Unio radiatus (Gmelin) to be suddenly with- 

 drawn, when subjected to the same experiment. 



I have ventured to name this family from 

 what I conceive to be the principal genus, after 

 repeated examinations of them. It is, however, 

 so difficult to determine the question of supe- 

 riority of structure, in animals whose organiza- 

 tion rests upon the same model, that the result 



