Synopsis Fresh-Wati;r Moij.usca 43 



r Surface tuberculous . 7 



6. • - 



[ Surface smooth or spiny 8. 



f Nacre deep purple Rotimdaria. 



[ Nacre white or tinged with pink Plethobasiis. 



(Hinge with perfect pseudocardinals and laterals 9. 

 Hinge teeth imperfect, vestigial Lastena. 



{Shell short, rounded, quadrate or oblique 10. 

 Shell (usually) elongate and straight ii. 



f Beak sculpture distinct, subconcentric, rounded upon the 



I posterior slope . .- Lcxingtonia. 



10. 1 



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

 looped Pleiirobcnia. 



r Beak sculpture running parallel with the growth-lines and 



11. ^ angled on the posterior slope Bll!t>tio. 



[ Beak sculpture concentric, rounded behind Uniomerns. 



Genus OUADRULA (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 Quadrula. 



Posterior slope with a radial furrow above the posterior ridge 



Quadrula s. s. 



No radial furrow above posterior ridge Theliderma. 



