240 ANNALS OF THE CARNEGIE MUSEUM. 
ex. Shell without sculpture. Soft parts more or less whitish, rarely 
slightly colored. Placentz whitish, rarely slightly colored. 
fi. Shell elongate and oblique, with the beaks placed rather an- 
teriorly, or rounded, squarish, or even elevated. Epi- 
dermis light, brownish, rarely dark brown, with or 
without rays. Nacre light colored. Beak-sculpture 
concentric, poorly developed............. Pleurobema. 
fs. Shell more or less elongate, but not oblique, beaks not much 
anterior. Epidermis dark or light, generally without 
rays, or rays indistinct. 
gi. Shell with rather dark epidermis, sometimes faintly 
rayed. Nacre often dark (pink to purple). Beak- 
sculpture concentric, with an angle upon the posterior 
ridge, but not double-looped, often faint and rudi- 
MENtAry ce cytiad GRC EEA OE se Elliptio. 
g. Shell with lighter epidermis (often with dark bands), rays 
practically absent. Nacre whitish. Beak-sculpture 
rather distinct, concentric, bars not angled behind, but 
regularly Cunvedsiip.e sacl aemietaoe ence css: Uniomerus. 
bs. Mantle-connection between anal and supra-anal openings well developed, 
but generally shorter than the anal. Shell not sculptured upon the 
disk, elongated, but not oblique. Beak-sculpture sharply double-looped 
or of the zig-zag type. Glochidia subtriangular, with hooks.. .Unio. 
a2. Beak-sculpture of the radial pattern. Mantle connection between anal and 
supra-anal present, rather long. Inner lamina of inner gills connected 
with abdomincal sac. 
b;. All four gills marsupial. Beak sculpture radial, well developed, more or 
less ‘extendins-upon the jdisky.c...c.c thie a ons hs cree tie tee eae Parreysia. 
bs. Two outer gills only marsupial. Beak-sculpture concentric-radial, rudi- 
TIVELUUATNG, foes eetalae. set Sayers: ste eget nan Ae lae RA eae eat ae ee eee ate ee Lamellidens. 
Genus FUSCONAJA Simpson. 1900. 
Simpson, 1900), p. 784 (as section). 
I consider this the most primitive type of the Unionid@ known to 
me. 
Shell simple, rounded, ovate, quadrate, or triangular, with more or 
less elevated beaks, well developed hinge-teeth, and rather deep beak- 
cavities. Outer surface without sculpture. Epidermis lighter or 
darker brown, with hair-like, dark rays, sometimes fused into spots 
when young. Beak-sculpture simple, concentric, slightly angled 
upon the posterior ridge, but not double-looped, not extending upon 
the disk, and often obliterated. 
Soft parts of primitive structure. Supra-anal separated from the 
