CONCHIFERA. 



289 



large, obtuse, round cardinal tooth to each valve, with 

 no lateral teeth.* Animal unknown. — Several species. 



Monocondylcea. 



Found in rivers of South America. 



Margaritana. Schum. (Alasmodonta Say.) — Shell 

 separated from Unio, as there is but one single side tooth 

 in each valve, or it is even entirely wanting ; in other par- 

 ticulars the same. Animal almost exactly like Unio. — 

 Many species. 



A considerable number of species are found in North 

 America, to which belongs the fresh water pearl mussel 

 ( Unio margaritifera), which in some countries of Northern 

 Europe occasionally contains very good pearls. 



Anodonta. Cuvier. — Shell generally thin, equivalve, 

 inequilateral ; hinge straight, with either no teeth or mere 

 rudiments ; shell transverse ; ligament external. Animal 

 oblong ; mantle freely open, with simple margins in front 

 and anteriorly, but fringed with numerous short cirrhi in 

 the branchial region, plain in the anal ; foot, large, broad, 



* Sowerbys Conch. Manual. 

 U 



