492 MEMOIRS OF THE CARNEGIE MUSEUM. 



64 pr. ct. of length). Valves not gaping. Dorsal margin nearly straight or very 

 gently convex, passing gradually into the anterior margin (rarely forming an in- 

 distinct angle). Posterior upper margin forming a blunt angle with the posterior 

 margin or passing gradually into it. Posterior margin obliquely descending, 

 more or less curved, and curving more strongly into the lower margin, without 

 forming a distinct posterior angle. The lower posterior end of the shell is rounded, 

 but hardly biangular. Lower margin in young specimens gently convex, in older 

 ones it is rather straight in the middle. Anterior end of shell not, or very little, 

 narrower than the posterior end. The shell is thus rather straight and not oblique. 



Valves only slightly convex, rather flattened upon the sides, with the posterior 

 ridge very indistinct and broadly rounded. Posterior slope somewhat compressed. 

 Greatest diameter near the middle of the shell, but not posterior to it. Beaks 

 not swollen, and not elevated, located at from 20 to 28 pr. ct. of the length. Beak- 

 sculpture consisting of about fourteen to sixteen rather sharp and fine, short, radial 

 bars, 5 to 7 mm. long, those upon the posterior ridge hardly longer than the rest, 

 and none of them uniting in tli(> middle of the shell with their lower ends. No 

 distinct granulations present, but sometimes there are a few irregular oblique 

 wrinkles upon the posterior slope near the beaks. A short, narrow lunula in older 

 shells. 



Epidermis in young specimens rather smooth and shining, but with fine, ir- 

 regular, concentric strise, nowhere lamellar. In old specimens it is less smooth, 

 chiefl}' on the posterior slope and toward the lower margin with more crowded 

 and rougher strise. Crinkled radial lines hardly indicated upon the shell. Color 

 in young specimens greenish bronze or brownish, sometimes with indistinct brown- 

 ish concentric bands, in older shells greenish tints disappear, and the epidermis 

 is dull brown or blackish brown. 



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

 tooth or slightly behind it, in older shells over its last third. Laterals curved, one 

 in right, two in left valve, somewhat rough posteriorly. Pseudocardinals normally 

 two in each valve. In j'oung specimens those of the right valve are obliquely and 

 forwardlj^ descending, comi^ressed, the anterior low and narrow, the posterior 

 higher and a little thicker, crenulated. In older specimens the posterior is thicker 

 and becomes generally more triangular and stumpy. Those of the left valve of 

 , young specimens are also compressed, but subtriangular and not very long, crenu- 

 lated, the anterior one larger than the posterior. In old shells these two teeth are 

 more stump}^, triangular, and not compressed. In rare cases reductions take place, 

 chiefly with regard to the anterior tooth of the right valve, or with regard to the 



