60S 



MEMOIRS OF THE CARNEGIE MUSEUM. 



compressed upon the posterior slope. Posterior slope with one or two blunt 

 radial ribs, which may be more or less distinct, or almost effaced. 



Epidermis rather smooth, with irregular concentric lines, which become sub- 

 lamellar upon the posterior slojie and near the lower margin. Traces of radial 

 sculpture are present in tlie shape of fine scalariform stripes, irregularly disposed 

 ujion the shell, and more or less numerous. Color of epidermis brownish or green- 

 ish. Upon the disk it may be quite green (dark or light), or it may be darker or 

 lighter brown, with irregular concentric bands of dark green. The posterior slope 

 is (in my specimens) always light brown, with or without greenish tints. No 

 color-rays are seen. 



Hinge-line gently curved. In the .younger siiecimens the part behind the beaks 

 is straight; in older ones it is gently curved, and curves down more distinctly under 

 the beaks. Ligamental sinus triangular, about as deep as wide, its anterior margin 

 vertical to the hinge-line. 



Cavity of shell and beaks moderate. Nacre -whitish, iridescent, only in the 

 youngest specimen with faint traces of radial strife. Prismatic zone very narrow 

 (comparatively widest in the young), subconcentric to the margin. Anterior 

 adductor-scar well impressed, chiefly in the older shells, subovate, united above 

 with the anterior retractor-scar. Anterior protractor-scar united with, or free 

 from, adductor-scar (this may be different in the right and left valves of the same 

 shell). Posterior adductor-scar less impressed, subovate, the posterior retractor- 

 scar forming an u])i)er triangular projection of it. Pallial line subconcentric to 

 lower margin. 



Me.\surements. 



' Length. 



Heiglit. 



Diameter. 



Beaks. 



Santa Rita. . . 52 mm. 33 mm. = 63 pr. cl. of L. 



Sapucay 54 



Santa Rita. . . 69 



33 



42 



= 61 

 = 61 



20 

 20 

 28.5 



= 38 pr. ct. of L. 

 = 37 

 = 41 



at 13 mm. = 25 pr. ct. of L. 

 13 " =24 

 16 " =23 



Lea's measurements for mortoniana give for the height 53 pr. ct. and for the 

 diameter 40 pr. ct. of the length. Thus the height is less here, but I think this 

 is due to the greater age of this specimen. My largest specimen has a peculiar 

 shape: the posterior end of the shell is drawn down, so that the lower margin is 

 almost straight. This, however, undoubtedly is an individual character, since the 

 growth-lines indicate that the young shell had the normal shape. 



The measurements for Hupe's shells are as follows : 



