Synopsis Fre;sh-Water Moli^usca 51 



Genus LEXINGTONIA Ortmann, 1914. 



"Shell subquadrate or subtrapezoidal, with slightly elevated beaks and 



well developed hinge teeth. Beaks not 

 -'""' '^ - much anterior. Outer surface without 



\^ sculpture. Epidermis lighter or dark- 

 er brownish, with rather indistinct 

 rays, which are narrower or wider 

 and do not break up into blotches. 

 Beak sculpture distinct, consisting of 

 rather numerous (six to eight), rather 

 crowded, subconcentric ridges, which 

 ■p- g form an indistinct, rounded angle up- 



on the posterior ridge and are in front 

 of this somewhat wavy and corrugated, but without showing any distinct 

 zigzag pattern. Towards the disk, they disappear. Nacre whitish or 

 pinkish." 



Animal having only the outer gills marsupial and subcyHndrical, red 

 placentae. 



Type: Unio subplanus Con., fig. 160. 



Genus ELLIPTIC Rafinesque, 1819. 



Shell inequilateral, ovate to elongated, rounded in front and pointed or 

 biangulate behind, with a more or less developed posterior ridge, often be- 

 coming slightly arcuate when old; beaks only moderately full, generally 

 sculptiired with coarse ridges, which run parallel with the growth lines or 

 are somewhat doubly looped, sometimes broken and showing fine radiating 

 lines behind ; surface smooth, slightly concentrically ridged or pustulous ; 

 epidermis generally rather dull colored, rayless or fully rayed; hinge-plate 

 narrow, two pseudocardinals and two laterals in the left valve and one 

 pseudocardinal and one lateral in the right, with rarely a vestige of a sec- 

 ond lateral; cavity of the beaks not deep or compressed. Marsupium occu- 

 pying the whole length of the outer gills only, forming a thick, smooth pad 

 when filled with young. ' 



Key to the sections of Blliptio. 



Shell spinose Canthyria. 



Shell smooth or feebly corrugated Blliptio s. s. 



