3- 



62 Bryant Walker 



Genus ALASMIDONTA Say, 1818. 



Shell generally rhomboid, inflated, with a well developed posterior ridge, 

 which ends in a point when it is single or a biangulation when double ; 

 beaks full and high, with coarse, concentric or slightly doubly looped bars ; 

 epidermis rayed, shining; hinge with two pseudocardinals in the left valve 

 and one in the right ; laterals usually wanting or imperfect, present in Pro- 

 lasinidonfa, cavity of the beaks deep; nacre bluish. Marsupium occupying 

 the entire outer gills. 



Key to the subgenera of Alasmido)ita. 

 ( Lateral teeth present Prolasmidonta 



[ Lateral teeth absent or obsolete 2 



f Shell solid, pseudocardinals solid, stumpv 3 



[ Shell thinner, pseudocardinals compressed or imperfect 4 



f Shell ovate-rhomboid, inflated Alasmidonta s. s 



Shell smaller, compressed, very solid Pegias 



Posterior slope slightly corrugated Riigifera 



Posterior slope smooth 5 



Shell rhomboid, posterior ridge low, rounded Prcssndojita 



Shell subtriangular, ])osterior ridge high, sharp .BullcUa 



Subgenus ALASMIDONTA s. s. 



Shell ovate-rhomboid, solid, inflated, 

 shining, with very strong, generally con- 

 centric, beak sculpture; pseudocardinals 

 solid, sttmipy, somewhat radiately ridg- 

 ed ; laterals short, very imperfect or 

 wanting; beak cavities deep, compressed. 



^. ,-, Type: Monodonta undidata Sav, fig. 180. 



Fig. 180 



Subgenus PRESSPDONTA Simpson, 1900. 



Shell small, decidedly rhomboid, surface gen- 

 erally painted with unbroken rays ; beak sculpture 

 slightly corrugated ; teeth compressed. 



Type: Unio calceohis Lea, fig. 181. Fig. 181 



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