Synopsis Fresh-Water Moli^usca 



69 



pressed; beaks curved inward and forward, their sculpture very faint, con- 

 sisting of slightly double looped ridges; sculpture of the shell nodular, 

 radiately wrinkled or lachrymose ; ligament black and conspicuous ; lunule 

 distinct, well developed ; epidermis shining, painted with a delicate mottling 

 on a light ground ; hinge plate wide and flat ; pseudo-cardinals heavy, tri- 

 angular, blunt and ragged ; laterals short, obliquely striated, cavity of the 

 beaks not deep ; nacre bright and silvery. 



Marsupium consisting of from seven to twenty-three very long, purple 

 ovisacs pendant from near the central base of the outer gills and formed 

 into a close coil with the ends turned inward. 



Type: Unio irroratns Lea, fig. 193. Animal, fig. 194. 



Genus PLAGIOLA (Rafinesque, 1819), Agassiz. 



Shell solid, surface irregularly, concentrically ridged; epidermis smooth- 

 ish, but here and there wrinkled ; painted with larger and smaller scattered 

 rays, which are generally broken into irregular lunate or squarish blotches ; 



Fig- 195 

 hinge heavy and strong ; hinge plate wide and flat ; female shell smaller than 

 the male, more inflated and swollen at the post basal region. Marsupium 

 large, projecting far below the inner gills. 



Type : Unio sccuris Lea, fig. 195. 



Genus AMYGDALONAIAS Fischer and Crosse, 1893. 



Shell inflated, decidedly truncate at 

 the posterior slope ; surface slightly con- 

 centrically sculptured; posterior ridge 

 sharp and well defined; epidermis shin- 

 ing, sometimes wrinkled, looped and 

 painted with 'a beautiful pattern of 

 broken or arrow-marked rays; area of 

 the beaks flattened off in the direction 

 of the axis of the shell, but not com- 

 pressed; beak sculpture delicate, some- 

 what broken and doubly looped, the anterior loop rounded, the posterior 

 sharp below, the ribs fading out where they cross the posterior ridge ; hinge 



Fig. 196 



