FAMILY NAIADES. 



217 



to this protean bivalve. A niollusk of large size, dwelling amid 

 such varied physical conditions as are presented by the places 

 which the Anodonta indiscriminately inhabits, would necessarily 

 produce a very variable shell, and some naturabsts appear almost 

 to think that each pond, canal, or river, has its peculiar species. 

 Even Linnaeus held to the opinion that we have two species in 

 Europe, one of oblong ventricose form, A. cygnea, and another of 

 oval compressed form, A. anatina, but these are simply the ex- 

 treme varieties. 



The shell of Anodonta cygnea is shortly rounded in front, and 

 towards the ventral margin, where the foot is exserted, it is a little 

 gaping. Posteriorly, where the pouted siphonal orifices protrude, 

 the shell slopes obliquely, and is sbghtly auricuJated. The species 

 is very closely represented in the United States by A.fiuviatilis. 



Unio tumidus. 



G-enus II. UNIO, Philippson. 



Animal ; elongately ovate, lobes of the mantle free, foot tongue- 

 shaped, rather large, branchial and excretory orifices approxi- 

 mating, edges of the mantle around the former moderately 

 fringed. 



Shell ; very inequilateral, inequivalve, anterior side short, a little 

 gaping where the foot is exserted, posterior side more or less 

 slopingly produced, valves rather thick, pearly, covered with a 

 stout horny epidermis, hinge composed of strong interlocking 



