I'RISODON I217 



its outline from the hinder basal point to the end of the post- 

 dorsal wing generally incurved; posterior slope often having 

 a radial row of plications, as in Cristaria; surface slightly con- 

 centrically sculptured and having" delicate radiating line 

 throughout, so that the surface is microscopically reticulated; 

 beaks full, but not elevated, without sculpture as far as 

 noticed ; epidermis yellowisli-green or bro'wnish, shining, often 

 bronzy ; hinge narrow under the beaks, widening in front and 

 behind ; left valve with two or more elongated, compressed 

 pseudocardinals, which show a tendency to break into denticles, 

 and two laterals ; right valve with two or more similar pseudo- 

 cardinals and a single lateral ; laterals vertically, granularly 

 striate. 



Animal with the labial palpi triangular, the hinder parts free 

 as in Unio, not united posteriorly ; branchiae large, equal in 

 size. (Troschel.) 



Type, Mya synnatophora Meuschen. 



The apparent want of beak sculpture, the smooth, slightly 

 decussated, shining and often metallic surface, the post-dorsal 

 plications, which when present, are similar to those of Cris- 

 taria, are, I think, characters of sufficient importance to justify 

 the separation of these species generically from Hyria. 



KUY TO SPKCFES OF PriSODON. 



Shell long, subrhomboid or subtrapezoid. 



Small, scarcely winged. P. hrownianus. 

 Large. 



Moderately winged. P. obliquus. 



Wings greatly produced horizontally. P. rectus. 



Shell short, subtrapezoid, sides compressed, P. complanatus. 

 Shell subtriangular. 



With a cord-like posterior ridge. P. syrmatophonts. 



Posterior ridge low. 



Wings small. P. castelnaudi. 



Wings large, the hinder elevated, P. alatus. 



Shell elliptical or obovate, compressed. P. ortonii. 



