14 

 FIRST SUB-FAMILY UniodioB. 



Shell transverse; cardinal tooth bilobed, an- 

 terior; lamellar tooth, posterior, horizontal, or 

 oblique; apices or beaks slightly oblique ; wrinkles 

 concentric or zonal. 



First Genus UNIO. 



Shell elliptical ; ligament straight ; cardinal 

 tooth bilobed, commonly furrowed or sulcated ; 

 lamellar tooth horizontal, often straight, never 

 flexuous ; axis variable ; marginal contour usu- 

 ally thickened ; three cicatrices or muscular 

 impressions. Animal with a large bilobed mantle, 

 not fringed ; syphons scarcely protruding ; with 

 a flat bilamellar appendage at the side of each 

 syphon ; branchiae striated in the form of a 

 second interior and bilobed mantle. 



It is thus I define the group, to which I retain 

 the name of Unio, because it appears to be the 

 most numerous, and resembles that known by this 

 name in Europe ; nevertheless it seems, that if 

 these shells are identically congeneric, the lamel- 

 lar appendages of the syphons, and the striated 

 mantle-like branchice, would have been noticed. 

 Besides the three muscular impressions, many 

 species have a muscular fossule or pit at the 

 extremity of the lamellar tooth, which, though 



