522 



MEMOIRS OF THE CARNEGIE MUSEUM. 



Epidermis somewliat sliining, witli uncniual, irregular, concentric wrinlcles, 

 more crowded and subkimellar upon the posterior slope and near the margins, 

 chiefl.y in older shells. Radial sculpture may be present, but indistinct, visible 

 chiefly upon the anterior part of the shell as " scalariform " stripes. Color of vyn- 

 dermis from yellowish brown to dark brown and blackish, but without any dis- 

 tinct traces of greenish tints. In young specimens, when well cleaned, the color 

 is generally yellowish or golden brown in the middle of the disk, shading to chestnut- 

 brown toward the ends and margins. Older shells are more uniformly chestnut- 

 brown to blackish brown (often coated with a dull black-brown deposit). Slight 

 traces of darker brown concentric bands are rarely present. 



Hinge-line generally distinctly curved. Ligamental sinus over the posterior 

 half or third of the laterals, generally very indistinct in old specimens. Lateral 

 teeth curved (less so in young shells), moderately long, one in right, two in left 

 valve, their edges and sides corrugated in old shells. Pseudocardinals directed 

 obliquely downward and forward, compressed and lamellar, thin in j'oung shells, 

 thicker in old ones. The right valve has nearly always two of them, equally high, 

 but the anterior narrower. The left valve has mostly only one; but there may be 

 a small and short posterior one, and even a small and narrow anterior one. In 

 one specimen (one out of over one hundred), the pseudocardinals are exactly re- 

 versed: one in right, two in left valve, while the laterals are normal. The edges 

 of the pseudocardinals are rugose and crenulated, but not dissected. 



Cavity of shell and beaks moderate. Nacre whitish, but very generally parti- 

 ally discolored, with irregular yellowish, brownish, or grayish spots. Anterior 

 adductor-scar impressed, rounded or sub triangular ; anterior retractor-scar separated 

 from it, rounded or irregularly oval, impressed, but not remarkably deep; anterior 

 protractor-scar connected with adductor-scar. Posterior adductor-scar ovate or 

 subtriangular, less deeply impressed; posterior retractor-scar generally separated 

 from it, but sometimes only indistinctly so. Pallial line distinct. Dorsal scars 

 few and irregular, in beak-cavity, forming an indistinct longitudinal row. 



Measurements. 



