582 MOLLUSCA 



A. imbecilis Say (Fig*. 914, D). Sliell very thin and fragile, ellip- 

 tical, smooth and shining, 59 mm. long, 29 mm. high, and 21 mm. wide; 

 dorsal margin straight ; bright grass green in color, with numerous darker 

 i"adial rays: valley of the Mississippi. 



4. Alasmidonta Say. Shell elliptical or rhomboidal, inflated, with 

 a well-developed posterior ridge; umbo rather prominent, with a few 

 coarse, parallel ri'dges; interior surface bluish; hinge with imperfect 

 teeth, 1 large cardinal being under the umbo in the left valve and a 

 smaller one in front of it and 1 cardinal in the right; laterals blurred: 

 15 species, all in North America, in muddy rivers, often buried several 

 inches in the mud. 



Key to the species of Alasmidonta here described: 



Gi Shell large. 



6i Height about half the length A. costata 



&2 Height nearly equal to length A. complanata 



a. Shell small A. undulata 



A. costata Rafinesque {A. rugosa Barnes). Shell elliptical, 16 cm. 

 long, 8 cm. high, and 46 mm. wide; surface with heavy lines of growth, 

 which form prominent wrinkles on the upper posterior portion; yellow- 

 ish-green in color, with delicate radial rays; cardinals thick: Mississippi 

 and St. Lawrence valleys. 



A. complanata Barnes. Hatchet back (Fig. 915). Shell very thick, 

 high, and irregular in shape, the posterior portion being quadrate or 

 triangular ; umbo with 4 coarse elevated ridges arranged in a double loop, 

 the apex of which points forwards ; color black or 

 brown, sometimes reddish; cardinal in right valve 

 very long; 15 cm. long; 11 cm. high; 5 cm. wide: 

 entire central part of continent, as far south as 

 Arkansas; common; sometimes contains pearls; 



Fig. 915-Alasmu "«^^ ^^ ^^^^^^ b"^^«^«- 



donta complanata A. undulata (Say) (Fig. 916, B). Shell short, 



ovate, inflated, dark green in color, with alternately 



yellowish and dark radiating rays ; umbo large and prominent, with 3 or 



4 concentric oblique undulations; 1 cardinal in each valve supported by 



a strong internal rib ; 50 mm. long ; 35 mm. high ; 25 mm. wide : Atlantic 



slope, south to South Carolina. 



5. Margaritana Schumacher. Shell large, elongated; hinge teeth 

 more or less imperfect and small; laterals sometimes wanting: 6 species; 

 circumpolar. 



M. margaritifera (L.) (Fig. 916, A). Shell more or less reniform, 

 thick, black in color; umbo not prominent; length 12 cm.; height 5 cm.; 

 width 3 cm.; cardinals, 2 large ones in left valve, the posterior grooved 



