Svxorsis Fresh-Watkr 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; glochidium 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 tendency toward the reduction of the hinge-teeth, and often they are 

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

 (Ortmann.) 



Kev to the genera of Anodontincr. 



f Beak sculpture concentric -■ 



[ Beak sculpture double-looped 4- 



2.-1 



r Beak sculpture fine Anodoutoides. 



} 



[ Beak sculpture coarse 3- 



r Pseudocardinals well developed Alasiiiidonta. 



[ Pseudocardinals rudimentary Strof^lntus. 



40' 



f Hinge whollv edentulous Anodonta. 



; 



I 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 Arcidens. 



I Beak sculpture poorly developed and not continuous with 



I surface sculpture Arkansia. 



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

 veloped Lasinigona. 



7-\ 



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



i each valve Simpsoniconcha. 



