544 MEMOIRS OF THE CARNEGIE MUSEUM. 



emargination of the i)ostorior margin corresponds to this depression. Diameter 

 40 to 46 pr. ct. of the length (according to Lea's figure a httle less, about 38 pr. ct.) 

 Beaks not swollen, and very slightly prominent beyond the hinge-line. Beak- 

 sculpture variable, but always more or less heavy, covering from 15 to 27 mm. of 

 the shell. There are about a dozen or more radial bars in front of the posterior 

 ridge, of which the anterior and posterior ones are narrow, while those in the 

 middle are thick and blunt. These ridges are very irregular; some anastomose, 

 smaller ones maj^ be intercalated between the larger ones, or they may be connected 

 laterally. In some specimens the beak-sculpture is much less developed, but 

 traces of the heavj^ ridges are always visible. Toward the lower margin, the beak- 

 sculpture, when well developed, stops suddenly. In addition, there are upon the 

 posterior slope a nimiber of fine, oblique wrinkles, but these are only well-developed 

 in young individuals. In the larger specimens there is a short lunula, which is 

 narrow or very narrow. 



Epidermis smooth, but with numerous irregular concentric wrinkles, which are 

 always crossed by more or less distinct radially impressed lines, forming here 

 and there low ridges. Where the beak-sculpture is well preserved, it is seen that 

 this radial sculpture is not a direct continuation of the beak-sculpture, but is in- 

 dependent of it, beginning before the latter ends. In fact, the beginning of this 

 sculpture causes, in part, the irregularities of the beak-sculpture. Color of epi- 

 dermis yellowish brown to dark brown, generally darker posteriorly, without any 

 distinct color markings. Sometimes there are greenish olive tints towards the 

 beaks. 



Hinge-line gently curved. Ligamental sinus shallow, over the middle of the 

 lateral teeth. Lateral teeth strong, moderately long, one in right, two in left 

 valve, descending posteriorly. In the youngest specimens, the upper margin is 

 somewhat elevated above the posterior ends of the laterals, while in the others 

 it is less elevated, almost parallel to them. Pseudocardinals strong, ragged, com- 

 pressed, running obliquelj^ forward and downward, straight or curved. In the 

 right valve there are two ]:)scudocardinals, the i)osterior higher and stronger, 

 much cut up. In the left valve there is one strong tooth, much cut up, and some- 

 times a much smaller one behind it, which, however, may be absent. There is 

 much variation in the raggedness of the pseudocardinals. 



Cavity of beaks and shell rather shallow. Nacre whitish or lurid, iridescent. 

 Anterior adductor-scar well impressed, subcircular, or subelliptical. Anterior 

 retractor-scar small, rounded, above the adductor-scar and separated from it. 

 Anterior protractor-scar rounded, connected with adductor-scar. Posterior ad- 



