584 



MEMOIRS OF THE CARNEGIE MUSEUM. 



iho lunula, and distinctly higher than the posterior upper margin, located at 28 to 

 33 pr. ct. of the length. Lunula cordiform, short and broad, almost as broad as 

 long. 



Epidermis dull, with crowded, irregular, concentric lines. Mj^ specimens 

 being all dead shells, the original structure is somewhat doubtful, but in some speci- 

 mens there are distinct indications of a lamellar, cloth-like structure. No radial 

 sculpture present, and no distinct radial rib upon the posterior slope. Color of 

 epidermis paler or darker brown, rather uniform, often with a more or less distinct 

 blackish ray upon the posterior slope, sometimes accompanied by a second ray. 



Hinge-line gently curved or almost straight behind the beaks. Under the 

 beaks it is sinuated, curving down more or less strongly, and then it is straight or 

 may curve up a little, rendering the lunula slightly concave. Pseudocardiiial 

 teeth of the usual type, but somewhat variable; that of the right valve in particular 

 may be more or less compressed. Ligamental sinus rather small, triangular, its 

 anterior margin obliciue. 



Nacre white, shining in well preserved shells, iridescent, and with rather dis- 

 tinct radial lines. Prismatic zone narrow, without any trace of a sudden widening. 

 Cavity of shell and beaks considerable. Anterior adductor-scar deeply impressed. 

 Anterior retractor-scar small, close to the adductor-scar, but distinctly separated 

 from it. Anterior protractor-scar united with adductor-scar. Posterior adductor- 

 scar faint, subovate, with a triangular upper process formed by the posterior 

 retractor-scar. No dorsal scars. Pallial line subconcentric to the margin. 



Measurements. 



Remarks. — There cannot be any mistake about this species, which differs from 

 all others by the greatly swollen valves and inflated beaks, which render the shape 

 almost subglobular. The diameter of 60 to 69 pr. ct. is not found in any other 

 species of the genus, and approached only by M. paruyuaijana, which, however, 

 distinctly differs in shape, being subangular, strongly oblique, and having the 

 posterior slope much compressed and subalate. In addition, the present species 

 differs from others by the narrow prismatic zone. It is impossible to give exact 

 figures for the location of the greatest height, as the height of the shell is essentially 

 the same for a considerable distance behind the middle. 



