578 ' MEMOIRS OF THE CARNEGIE MUSEUM. 



higny's figure, and hardly ronninji; a point antl an cniargination on the iwsterior 

 margin, as in this figure). Color of epidermis dark grayish green to bLackish, the 

 grayish green shade due to tlie preservation of the lamella?. A rather distinct 

 black radial ray upon the rib of the jiosterior slope, and faint traces of additional 

 rays in front of and behind it-. 



Hinge gently curved behind the beaks. Under the beaks it is sharply curved 

 down, and in front of them it curves up again, thus becoming concave. Each 

 valve has a single pseudocardinal tooth of the typical shape ; that of the left valve 

 anterior to, and a little below, that of the right; both triangular and depressed, 

 and projecting under the margin of the opposite valve, where they fit into shallow 

 grooves. Ligamental sinus triangular, wider than deep, its anterior margin oblique 

 to the hinge-line. 



Nacre very iridescent, whitish, with a faint salmon blush in the cavity of 

 the shell, and purple and greenish reflections toward the margins. Irregular 

 radiating lines are present toward the margin. Prismatic zone of unequal width, 

 rather narrow behind, widening quite suddenly at the lowest point of the shell, 

 and remaining wide along the anterior ascending part of the lower margin, narrowing 

 again at the anterior end. Color of prismatic zone grayish green. 



Cavitj' of shell and beaks moderate. Anterior adductor-scar deep, elliptical. 

 Anterior retractor-scar small, at the upper end of the adductor-scar, distinctly 

 isolated in left valve, narrowly connected with it in the right. Anterior protractor- 

 scar small and united with adductor-scar. Posterior adductor-scar faint, subovate, 

 with an upper triangular projection formed by the posterior retractor-scar. No 

 dorsal scars. Pallial line subconcentric to the margin. 



Measurements. 



Sex. Length. 



Height. Diameter. 



Beaks. 



Greatest Height. 



9 I 53 mm. !43 mm. =81 pr. ct. of L. 31 mm. =59 pr. ct. of L. at 16 mm. =30 pr. ct. of L. | at 30 mm. =57 pr. ct. of L. 



Remarks. — In general outline this species is much like M. lentifonnis, as is 

 shown by the proportion of height to length and the location of the greatest height 

 of the shell. It markedly differs, however, in the much more swollen shell (diameter 

 59 pr. ct. as against 41 to 51 \n\ ct. in lentiformis), and the more inflated beaks. 

 In consequence of the very convex disk, the posterior slope appears much more 

 compressed and alate. 



Anatomy. — The specimen at hand for examination is a barren female. 



The structure is absolutely identical with that of M. lentiformis, and nothing 

 is to be added, except that the crenulations of the lower part of the anal ojiening 



