24 Lamarck's Genera of Shells. 



The Naiada are well distinguished from the fresh water 

 conchse by their hinge, and the animal inhabitant. The shell 

 is free, regular, equivalve, inequilateral, always transverse ; the 

 epidermis is greenish, inclining to brown, and is always wanting 

 at the beak. The muscular impressions are lateral, and quite 

 separate ; that of the posterior side is composed of two or three 

 distinct or unequal impressions, which distinguishes them from 

 the other bimuscular conchifera. 



The animal has no projecting syphon or tube ; its foot is 

 lamellar, transversely elongated, and rounded, which it pro- 

 trudes beyond the valves, and uses for locomotion. It generally 

 remains partly buried in the mud, with the beaks immersed. 



1. Unio*. 



Shell transverse, equivalve, inequilateral, free ; beaks decor- 

 ticate, almost eroded. Muscular impression posterior, com- 

 pound. Hinge with two teeth on each valve; one, cardinal, 

 short, irregular, simple, or bifid, substriated ; the other elon- 

 gated, compressed, lateral, channelled, extending under the 

 corselet, for a considerable space along the lower margin on that 

 side. Ligament external. 



Linnaeus confounded the Unio with the Mya, although the 

 latter is a sea shell, and very different in form, hinge, position 

 of the ligament, and the animal which inhabits it. 



The Unio is eminently distinguished from the Anodonta, 

 (which it resembles externally,) by its hinge. Each valve has 

 a short cardinal tooth, that on the left valve generally simple, 

 that on the right divided into two lobes, besides a lateral tooth, 

 as described above. The two testh of each valve articulate toge- 

 ther when the valves are shut. The shell of the Unio is formed 

 in general, of a very brilliant mother-of-pearl ; externally, it is 

 covered with a greenish or brown epidermis, except on the 

 beaks, which are decorticate, and more or less carious. Lastly, 

 the lamina of the margin of the shell, above the lateral tooth, 

 bas a truncation or sinus, which seems to receive a portion of 



f A pearl called an Union, from unus, because no two, found in the same 

 shell, are alike. 



