182 MEMOIRS OF THE CARNEGIE MUSEUM. 



times rather distinct, so as to render the bars almost double-looped, sometimes 

 hardly indicated, so that the middle part of the bars is almost straight. The 

 fourth bar, if present at all, is rudimentary, and only its middle part is distinct. 

 Radiating lines upon the posterior slope indistinct or absent. But upon the 

 posterior slope there is a system of fine, irregular, radial wrinkles or ridges, running 

 toward the upper posterior margin. These wrinkles are rarely absent or obscure, 

 but generally well-developed; they may be finer or coarser, and may extend over a 

 larger or smaller portion of the posterior slope; generally they are less distinct 

 toward the posterior end in old shells. 



Shell inflated, greatest width at or immediately in front of the posterior ridge. 

 Lateral faces gently convex, with a very distinct posterior ridge, which, although 

 rounded, is well marked in consequence of the truncated character of the posterior 

 slope. Very often the posterior ridge appears as if bounded by two ridges, includ- 

 ing an elevated part of the shell between them : this is chiefly the case toward the 

 posterior end of old shells. The posterior slope is somewhat compressed and 

 elevated in the middle, but in its general shape it presents a truncate, in some 

 cases almost flat appearance. 



Epidermis yellowish, greenish, or brownish, blackish in old shells, generally 

 with very distinct greenish or blackish raj^s. Rays narrow or wide, straight, 

 in most cases more or less broken up into (or overlaid by) darker spots. The 

 spots are best developed towards the beaks, but often they are distinct all over 

 the shell, and very rarely entirely wanting. Generally, the posterior slope is 

 lighter in color than the rest of the shell, and has fewer and less distinct rays and 

 spots. Often there are also lighter and darker concentric bands, marking, more 

 or less, the growth-rests. In old shells, the color pattern becomes obscure, and 

 the shell is more or less uniformly blackish. 



Hinge incomplete. Pseudocardinals rather thin, but distinct, depressed- 

 triangular (not stumpy), one in each valve, and the left valve has, in addition, a 

 more or less well-developed interdental tooth, which may be separated from the 

 pseudocardinal, or more or less united with it. More rarely it is rudimentary. 

 Lateral teeth absent. Beak-cavity moderate. Dorsal muscle-scars in the beak- 

 cavity upon the hinge-plate. Adductor-scars distinct and rather deeply impressed 

 anteriorly, less so posteriorly. 



Nacre bluish white, sometimes in places greenish or grayish, discolored, 

 iridescent posteriorly, rarely with very pale salmon tints, never distinctly pinkish 

 or reddish. 



Shell of the female hardly distinguishable from that of the male. Some 



