Synopsis Fresh-Water Moli^usca 55 



charged,, and the thickened tissue at the edge capable of stretching out in a 

 direction transverse to the gill, but not beyond the edge (or only slightly 

 so) ; water-tubes in the gravid female divided longitudinally into three 

 tubes, with only the one in the middle used as an ovisac, and closed at the 

 base of the gill; glochiditmi semicircular or triangular, with a spine (hook), 

 in the middle of the ventral margin of each valve; shell generally very 

 heavy, often thin, never round, but more or less elongated ; color of epi- 

 dermis generally bright and with color markings ; sculpture of the beak 

 double-looped or concentric, in the latter case often extremely heavy ; hinge 

 rarely complete and, if so, of peculiar structure ; generally there is a dis- 

 tinct tendenc}^ toward the reduction of the hinge-teeth, and often they are 

 completely absent; sexual differences in the shell very rarely present." 

 (Ortmann.) 



Key to the genera of Anodontincv. 

 { Beak sculpture concentric ■ • ■ 2. 



I, 



.J 



[ Beak sculpture double-looped 4- 



r Beak sculpture fine Anodonf aides. 



2.-1 



[ Beak sculpture coarse 3- 



r Pseudocardinals well developed AJasuudonta. 



y Pseudocardinals rudimentary Strophitus. 



3- -^ 



A.\ 



f Hinge wholly edentulous Anodonfa. 



[ Hinge teeth more or less developed 5- 



f Beak sculpture tubercular ; surface tubercular or folded 6. 



5.-] Beak sculpture not tubercular; surface smooth except on 



[ posterior slope 7- 



f Beak sculpture strong and continuous with tubercular sur- 



I face sculpture , Arddens. 



I Beak sculpture poorly developed and not continuous with 



j surface sculpture Arkansia. 



Beak sculpture double-looped ; pseudo-cardinals fully de- 

 veloped Lasmigoua. 



I Beak sculpture open behind ; a single pseudo-cardinal in 



i each valve Siuipsoniconcha. 



