96 THE CHICAGO ACADEMY OF SCIENCES. 
rounded before and broadly elliptical behind, the female broader 
than the male; dorsal and ventral margins curved; surface 
smooth and shining; growth lines prominent near the ventral 
border; umbones not elevated, inflated, dark reddish brown in 
color, and marked by five elevated, undulating ridges, causing 
the umbo to resemble a miniature Quadruda plicata; anterior um- 
bonal slope rounded, posterior slope broadly rounded; ligament 
short, wide, stout, dark chestnut color; epidermis yellowish 
green, tinged with dark red, and marked by very many dark 
green rays, which are wavy in outline and vary greatly in width; 
cardinal teeth double in both valves, nearly equal in the left, 
and the anterior tooth very small in the right, elevated, triangu- 
lar, serrated on the edge, all pointed forward; the anterior tooth 
in the right valve is indicated only by a thickening of the mar- 
gin; lateral teeth short, thin, lamelliform, more elevated in the 
center than at each end, striated; anterior adductor muscle scar 
much longer than wide, very deeply impressed, coarsely striated; 
posterior adductor muscle scar rounded, as wide as long, not 
deeply impressed, concentrically striated, confluent; protractor 
pedis muscle scar wider than long, deeply impressed, striated, 
longer at the anterior than at the posterior end; dorsal muscle 
scars situated in the cavity of the beaks and on the posterior 
face of the cardinal teeth, numerous, deeply pitted; cavity of the 
beaks rather deep; pallial line very wide, impressed; nacre sil- 
very white, iridescent, especially on the posterior part, showing 
the rays through the shell on the ventral and posterior borders; 
the center of the shell has what the silversmiths would call a 
‘¢satin finish.” 
Length, 52.00; height, 37.00; breadth, 23.00 mill. ¢ (7502). 
“60.00; ‘ 45.00; « 30.00 ‘* 9 (7503). 
he 43.00; ** 32.00; “ B0O0 =e" © (7501). 
Animal: Anal opening rather large, with dark papille; 
branchial opening large, with coarse, dark papille; mantle bor- 
der greatly thickened and dentate below; inner gill much the 
larger in front, united to the abdominal sac throughout; mar- 
supium occupying the posterior half of the outer gill of the fe- 
male, ovisacs large, dark below. (Simpson. ) 
Distribution: Mississippi and Ohio valleys and Lake 
Michigan. 
Geological distribution: Pleistocene. 
Habitat: Sameas L. iris. 
