42 Br V A XT WalkKr 



Key to the subfamilies of Unionidcr. 



[ Water-tubes simple in the gravid female v 



\ Water-tubes in the gravid female divided 'into three tubes, 



[ of which only the centre one is used as an ovisac Anodonfiucc. 



Male and female shells usually alike ; edge of the gravid 



marsupium always sharp and not distending UnionUicc. 



2. \ Male and female shells usually different ; t(\g& of the gravid 

 marsupium distending and bulging out beyond the original 

 [ edge of the gill LarnpsUincr. 



Subfamily UNIONIN^ (Swainson. 1840) Ortmann, 1910. 



"Inner lamina of the inner gills generally free from the abdominal sac 

 (sometimes, in extralimital forms, connected) ; supra-anal opening some- 

 times not separated from the anal, normally present, the closed part rather 

 short ; branchial opening well-defined ; no papillae nor flaps on the edge of 

 mantle in front; marsupium formed by all four gills or by the outer gills 

 only; edge of marsupium always sharp and not distending; water-tubes not 

 divided in the gravid female ; glochidium semielliptic or semicircular, with- 

 out spines; shell generally heavy and solid, rounded to elongated, mostly 

 with dull-colored epidermis ; sculpture of the beak generally rather indis- 

 tinct, concentric or pustulous or with indications of double loops or zig- 

 zag bars ; hinge always complete, with rather strong teeth ; generally no 

 difference of sex shown in the shell." (Ortmann.) 



Key to the genera of Unionbuv. 

 r All four gills serving as marsupia 2 



[ Outer gills only serving as marsupia 6 



f Male and female shells alike 3 



[ Male and female shells different Tntogonia 



f Hinge with perfect pseudocardinals and laterals 4 



[ Hinge teeth rudimentary or wanting Gonidca 



{ Surface plicate c; 



4. \ Surface pustulose Ouadrula 



[ Surface smooth Fusconaia 



Beaks sculptured with coarse, concentric or somewhat dou- 

 ble-looped ridges, which do not extend over the suria.ce. A inbl ana. 



5-1 



Beaks sculptured with strong, zig-zag ridges extending over 



the upper surface Megalonaias. 



