Hinge plate moderate to heavy; shell slightly over two times as long 

 as high 69. 



69. Shell rhomboid; dorsal margin meeting the posterior margin at an angle; 



hinge plate moderate; nacre purplish, often coppery tinted; Atlantic 

 drainage 



Elliptio complanatus (Dillwyn) Spike 

 Shell elliptical; dorsal margin sloping smoothly into the posterior mar- 

 gin; hinge plate thick and heavy; nacre usually purple; most of U. S. 

 except the Pacific states 



Elliptw dilatatns (Raf.) Lady-finger 



70. Shell with a heavy^ hinge plate, with the larger cardinal tooth in the left 



valve directed toward the posthasal angle and almost parallel with 



the laterals; outline of shell almost circular or triangular 71. 



Cardinal teeth slender or directed downwards or anteriorly at almost a 



right angle or more with the laterals; outline various 83. 



71. With a long, flat posterior slope 72. 

 Posterior ridge faint or double; posterior slope rounded or low 74. 



72. Umbones full; epidermis often with broken rays (color markings); east' 



ern and east-central states 



Pleurobema viytiloides Raf. Club Shell 

 Pleurohema clava (Lamarck) 

 Umbones flattened 73. 



73. Epidermis yellowish, with rays which may be broken up into squarish 



blotches; nacre white; central and east-central states 

 Plagiola lineolata (Raf.) Butterfly Shell 

 Epidermis dark; nacre purplish; Mississippi drainage 

 Elliptio niger (Raf.) Elephant's Ear 

 Elliptio crassidens (Lamarck) 



74. Nacre usually purplish 75. 

 Nacre usually w-hite or pink 76. 



75. Shell almost circular, except for the very prominent beaks; no radial 



swelling or depression; Ohio, Cumberland and Tennessee drainages 

 Obovaria retusa (Lamarck) Golf-stick 

 Shell more angular; usually with a radial swelling or depression before 

 the posterior ridge; Ohio River drainage 

 Dysnomia sulcata (Lea) Cat's Paw 



76. Lunule not extending before the beaks, with the anterior margin scarcely 



or not projecting forward 77. 



Lunule projecting to a point well before the beaks 78. 



77. Beaks shallow; posterior muscle scars D-shaped; with a small anterior 



cardinal in the right valve; east-central and central states 



Obovaria olivaria (Raf.) Hickory-nut 

 Beaks narrowed under cardinals; posterior muscle scars almost circular; 

 no anterior cardinal in the right valve; Mississippi drainage 



^uadrula antrosa (Raf.) Nigger-head 



Fusconata ebenus (Lea) 



157 



