326 MEMOIRS OP THE CARNEGIE MUSEUM. 



moderately swollen, and moderately elevated above the hinge-line, located in 

 front of the middle of the shell, but not very near the anterior end. Beak-sculpture 

 rudimentary, consisting of about three faint, indistinct bars, which have a tendency 

 to fall into two loops. Valves greatly swollen, but rather flat upon the sides, convex 

 anteriorly, and with a very distinct and strong, but blunt, posterior ridge; no 

 furrow or depression in front of the latter. Behind the posterior ridge the posterior 

 slope is sharply truncate and flat, being very little elevated in the middle. 



Epidermis yellowish, yellowish green, or light green, always with well developed 

 dark green to blackish raj's, which are distinct, broad and bold, and generally 

 have the tendency to become dissolved into squarish, triangular, or sagittate spots. 

 In most cases the rays cover all of the lateral faces of the shell, but they are in- 

 distinct or entirely absent on the posterior slope. In old specimens the rays 

 disapijear toward the lower margin. Concentric bands of color are indicated 

 chiefly by a tendency to have the spots of the rays arranged in concentric lines. 



Hinge well-developed. Pseudocardinals generally two in each valve, but the 

 anterior tooth in the right valve is smaU. The teeth are strong, stumpy, ragged. 

 Interdentum absent. Laterals of medium length, strong. Beak-cavity moderate. 

 Dorsal muscle-scars in beak-cavity. Adductor-scars distinct, and anteriorly 

 strongly impressed, less so posteriorly. Nacre alwaj's silvery white. 



Sexual differences of the shell very strongly marked. The male has a rather 

 regular, subtriangular outline, with the posterior ridge moderately elevated, the 

 greatest diameter of the shell situated slightly in front of the ridge, and with a 

 moderately broad posterior slope, without sculpture, and with the edge of the 

 shell entire. In the female the marsupial expansion is located on and restricted 

 to the posterior ridge. In consequence of this the posterior ridge is greatly ele- 

 vated, and the greatest diameter of the shell is located upon the posterior ridge, 

 and in old females the diameter of the shell often exceeds the height of the shell 

 (a proportion quite unique among Naiades). The posterior slope in the female is 

 greatly enlarged, and in addition there is a sculpture of radial ribs upon the posterior 

 ridge and posterior slope, which produces a denticulation on the margin of the 

 shell in this region. This sculpture may be present, in a rudimentary condition, 

 in old males. The general outline of the female shell is a little different, the pos- 

 terior end projecting slightly beyond the line of the lower margin, so that there 

 is a slight concavity of the lower margin in front of the posterior end. The female 

 shell does not nearly attain the size of the male shell. 



