Synopsis Fre;sh-Wate;r MolIvUSca 51 



Genus- LEXINGTONTA 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 

 TT- ,, 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 subcylindrical, red 



placentae. 



Type: Unio sitbplajiiis 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 

 sculptured 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-plat<^ 

 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 Elliptio. 



Shell spinose Canthyria. 



Shell smooth or feeblv corrugated BlUptio s. s. 



