62 



Bryant Wai,ke;r 

 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- 

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

 the entire outer gills. 



Key to the subgenera of Alasmidonta. 

 r Lateral teeth present Prolasmidonta. 



[ Lateral teeth absent or obsolete 2. 



f Shell sohd, pseudocardinals soHd, stumpy 3- 



[ Shell thinner, pseudocardinals compressed or imperfect. 4. 



C Shell ovate-rhomboid, inflated Alasmidonta s. s. 



[ Shell smaller, compressed, very soHd Pegias. 



{ Posterior slope slightly corrugated Rugifera. 



Posterior slope smooth 5- 



Shell rhomboid, posterior ridge low, rounded Pressodnnta. 



Shell subtriangular, posterior ridge high, sharp Biillella. 



Subgenus ALASMIDONTA s. s. 



Shell ovate-rhomboid, solid, inflated, 

 1 shining, with very strong, generally con- 

 centric, beak sculpture; pseudocardinals 

 * solid, stumpy, somewhat radiately ridg- 

 ed ; laterals short, very imperfect or 

 Avanting; beak cavities deep, compressed. 



.. ^ Type: Monodonta nndidata Say, fig. 180. 



Fig. 180 



Subgenus PRESSODONTA Simpson, 1900. 



Shell small, decidedly rhomboid, surface gen- 

 erally painted with unbroken rays ; beak sculpture 

 slightly corrugated; teeth compressed. 



Type: Unio calceolus Lea, fig. 181. 



Fig. 181 



