Synopsis Fresh-Water Moixusca 43 



{Surface tuberculous 7 

 Surface smooth or spiny -. 8. 



f Nacre deep purple Rotimdaria. 



I Nacre white or tinged with pink PlctJiohasiis. 



{Hinge with perfect pseudocardinals and laterals 9. 

 Hinge teeth imperfect, vestigial Lastcna. 



{Shell sliort, rounded, quadrate or oblique lO. 

 Shell (usually) elongate and straight 11. 



Beak sculpture distinct, subconcentric. rounded upon the 



posterior slope l.cxingtonia. 



\o.\ ' 



I Beak sculpture coarser, inclined to be more or less double- 



[ looped Pleiirobcma. 



( Beak sculpture running parallel with the growth-lines and 



II. -{ angled on the posterior slope Ellipfio. 



[ Beak sculpture concentric, rounded behind Uniomcrns. 



Genus QUADRULA (Rafinesque, 1820) Agassiz. 



Shell triangular, quadrate or rhomboid ; solid, inflated with rather coarse 

 prominent beaks, which are generally sculptured with a few coarse, irreg- 

 ular, subparallel ridges that are inflated where they cross the posterior ridge ; 

 posterior ridge ordinarily well developed; disk sculptured or smooth; epi- 

 dermis usually dull-colored, dark and rayless or feebly rayed ; hinge plate 

 heavy, wide, flattened ; pseudocardinals solid, direct, ragged ; laterals double 

 in the left and single in the right valve ; cavity of the beaks deep and com- 

 pressed. Marsupium occupying all four of the gills throughout, the whole 

 smooth and pad-like. 



Key to the sections of Onadrnla. 



Posterior slope with a radial furrow above the posterior ridge 



Quadrula s. s. 



No radial furrow above posterior ridge Theliderma. 



