OF THE FAMILY OF NAIADES. 405 



dermis beyond the margin of the shell seems well adapted, 

 when the animal closes the valves, to exclude the entrance of 

 water, &c., and doubtless is thus used. 



When a conchologist examines a shell which to him is 

 new, almost the first question he puts to himself is, " what 

 must be the form of the animal which once inhabited this 

 covering?" He judges by analogy; and after examining the 

 form of the shell, he has generally a very good idea of its for- 

 mer inhabitant, and although he may not be able to decide 

 with the same precision as the osteologist, he can place it in 

 its proper family. 



Each family has a form of shell adapted to the^ wants of 

 its inhabitant, and peculiarly fitted for its locomotion or its 

 fixed situation. Thus the Ostracea could not exist in the 

 shells of the Naiades, although the forms of the animals are 

 not very dissimilar to the unpractised eye. The naturalist, 

 however, sees in the former the entire want of the muscular 

 foot for locomotion and its attendant pairs of muscles. In the 

 valve of this he sees but one muscular impression, which 

 muscle is used for the sole purpose of closing the valves, while 

 in the other he sees at least four, two of the muscles of which 

 are used for protruding, the other two for retracting the foot 

 by which it propels itself. The species of the family Myti- 

 lacea attach themselves by a strong byssus to stones, &,c., and 

 therefore require a very differently constructed shell. The 

 Lithophaga bore into stone, wood, mud, &c., and have no 

 power of locomotion. The Solenacea generally live in pits, 

 and move only between the two extremities of them. To 

 these families might be added many more, all of which are 

 as different in form and habits, as can well be imagined. It 

 may therefore be safely asserted that the student of concho- 

 logy can always form some idea of the animal from the form 

 of the shell. 



My attention having been particularly drawn to the study 

 of the family of the Naiades, and my cabinet possessing a 

 great number of species and varieties, I feel induced in this 

 preliminary matter to say something on the species of the 



