240 Annals of the Carnegie Museum. 



er. Shell without sculpture. Soft parts more or less whitish, rarely 

 slightly colored. Placentae whitish, rarely slightly colored. 

 fu 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. 



/j. 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 Elliptic 



g-2. Shell with lighter epidermis (often with dark bands), rays 

 practically absent. Nacre whitish. Beak-sculpture 

 rather distinct, concentric, bars not angled behind, but 



regularly curved up Uniomerus. 



b->. 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. 

 ai. 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 extending upon the disk Parreysia. 



b-.. Two outer gills only marsupial. Beak-sculpture concentric-radial, rudi- 

 mentary Lamellidens. 



Genus Fusconaja Simpson. 1900. 

 Simpson, 19006, p. 784 (as section). 



I consider this the most primitive type of the Vnionidcc 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 



