Synopsis Fresh-Wate;r Moli^usca 



57 



Marsupium occupying the whole outer gills, when filled 'forming a 

 smooth, YCT}' thick, liver-colored pad. 



Type : Mytilus cygneiis h. 



Example: A. cataracta Say, fig. 170. Animal, fig. 171. 



Fig. 171 



Genus ANODONTOIDES Simpson, 1898. 



Shell elliptical, inflated, thin, with a faint posterior ridge, sometimes 

 constricted at the centre of the base; beaks rather full, with a few, not very 

 coarse, subparallel, concentric ridges, which are curved up rather suddenly, 

 behind and back of these are fine radiating ridges ; epidermis smooth and 



Fig. 172 



shining, often rayed; hinge line slightly incurved in front of the beaks, 

 edentulous or bearing the merest rudiments of teeth; nacre bluish-white. 

 Marsupium occupying the outer gills only. 



Type: Anodonta ferussaciana Lea, fig. 172. 



Genus ARCIDENS Simpson, 1900. 



Shell subsolid, inflated, subrhomboidal, with full high beaks ; beak sculp- 

 ture very strong, consisting of irregular corrugations, which fall into two 

 loops, at the base of which the ridges are swollen into knobs that continue 

 out in two radiating rows on to the disk of the shell ; in front of and be- 

 hind the beaks are many fine, radial wrinkles, the posterior ones being zig- 



