580 MEMOIRS OF THE CARNEGIE MUSEUM. 



very gently curved or almost straight, sloping ui)vvard; the lowest point located 

 rather far backward (at 66 to 77 pr. ct. of the length). Anteriorly and posteriorly 

 the lower margin curves up into the anterior and posterior margins. Anterior 

 end of shell markedly narrower than the posterior. 



Valves very convex, but distinctly flattened upon the sides of the disk, which 

 flattening may even become a shallow depression. Greatest convexity in front 

 and behind the flat area, and the posterior convexity, forming the rounded pos- 

 terior ridge, is the stronger. Posterior slope somewhat compressed and elevated, 

 but hardly alate. At its anterior end the shell is also a little compressed. Diameter 

 50 to 56 pr. ct. of length, but sometimes falling below 50 pr. ct., and such specimens 

 approach the var. jxirchappi. Beaks more or less swollen, inflated, and incurved, 

 prominent over the lunula, but onl}' little elevated above the posterior part of 

 the upper margin. Lunula distinct, short, elongated triangular, variable, narrower 

 in young, broader in old specimens, where it may be half as wide as long. 



Epidermis dull, cloth-like. Fine and crowded, anastomosing, concentric 

 strise are elevated as fine lamellae, but often they are abraded. No radial sculpture 

 visible, and there is hardly a trace of a radial ridge upon the posterior slope. Color 

 of epidermis dark grayish green, without any trace of color-markings, except oc- 

 casionally a mere indication of a dark ray upon the posterior slope. 



Hinge straight, or gently curved, in its posterior part. Under the beaks it 

 curves down more or less suddenly, and then becomes straight again, or even some- 

 what concave. The curvature of the hinge-line is quite variable, and corresponds 

 to the varying degree of the development of the beaks. Pseudocardinal teeth 

 as in M. paraguayana. Ligamental sinus triangular, wider than deep, with its 

 anterior margin oblique to the hinge-line. 



Nacre very iridescent, whitish, with greenish and jiurplish tints, chiefly pos- 

 teriorly. Irregular radiating lines present, but faint in old specimens. Pris- 

 matic zone of unequal width, rather narrow behind, suddently widening at the 

 lower point of the lower margin, and remaining wide to near the anterior margin. 

 However this widening is not so strongly' pronounced as in M. paraguayana, and 

 is also individually variable. Color of jirismatic zone greenish gra3\ 



Cavity of shell and beaks moderate. Anterior adductor-scar sharpl\' and 

 rather deeply impressed, subelliptical; anterior retractor-scar small, connected with, 

 or separated from, adductor-scar; anterior protractor-scar connected with adductor- 

 sear. Posterior adductor-scar distinct, but less strongly impressed; scar of the 

 posterior retractor forming a short, triangular upper process of it, which, in a few 

 specimens, is jiartly separated from it. No dorsal scars. Pallial line distinct, 

 subconcentric to the margin. 



