SvNorsis Frksh-Water Moli^usca 



63 



Subgenus PROLASAIIDONTA Ortmann, 1914. 



Lateral hinge teeth present, but their number re- 

 versed, two in tlie right, one in the left valve. Beak 

 sculpture moderately heavy, bars with an angle upon 

 the posterior ridge and a slight sinus in front of it. 

 Inner lamina of inner gills free. Female shell recog- 

 nizable by a slight swelling in the region of the pos- 

 terior ridge. Fig. 182 



Tyj^e : Uiiio hctcrodon Lea, hg. 182. 



Subgenus PEGIAS Simpson, 1900. 



Shell small, thickened in front, with a sharp 

 posterior ridge, in front of which is a wide, radial 

 impression, ending in a basal sinus ; above this 

 ridge is another, making the shell decidedly biangu- 

 late and truncate behind; beak sculpture consisting 

 of sul)-conic corrugations, generally swollen on the 

 ])Osterior ridge ; epidermis decorticated, but showing a few, dark radial 

 rays on the base of the shell; pseudocardinals rather solid; laterals wanting. 



Type: Morgaritana fabula Lea, fig. 183. 



Subgenus RUGIFERA Simpson, 1900. 



Fig. J 84 



Shell elongated, rhomboid, inflated, surface brilliantly painted with radi- 

 ations which often break into dappled or splashed pattern of color; pos- 

 terior slope slightly corrugated ; teeth very imperfect ; laterals wanting. 



Type: Alasmodonta marginata Say, fig. 184. 



