LAMELLIBRANCHIATA. 245 



becoming gibbous and arcuate above. Right valve concave below, flat in the 

 middle and dejiressed-convex in the umbonal region. 



Hinge-line straight, less than the length of the valve. 



Beak of left valve prominent, anterior to the middle of the valve, acute, 

 inclined forward and arching over the hinge-line. Umbonal region promi- 

 nent, gibbous, abruptly limited on either side. In the right valve the beak 

 does not rise above the cardinal line. Umbonal region defined on the ante- 

 rior side by a distinct oblique fold or ridge, and on the posterior side by a 

 well-marked depression limiting the wing, subtending an acute angle. 



Ear small, limited by a shallow sinus below. Wing large, triangular, 

 extending more than half the height of the shell toward the base ; margin 

 gently concave and slightly recurving toward the hinge-line ; extremity 

 angular. 



Test of moderate thickness, marked by somewhat distant, rounded, 

 abruptly elevated radii, with wider intermediate flat spaces which are some- 

 times marked by one or more smaller rays. In the pa,rtial cast, these rays 

 are crossed, and the intermediate spaces cancellated, by fine concentric striae. 

 Towards the ventral and baso-lateral margins the shell is lamellose and the 

 radii become obsolete. The concentric strife are crowded upon the wing, 

 and the radii are less conspicuous than on the body of the shell. 



Ligamental area in left valve large ; smaller in the right valve. The pallial 

 line in the left valve forms a distinct nodose ridge, which extends from the 

 rostral cavity in a slightly arcuate line to below the middle of the valve. 

 In the right valve, the pallial line in the cast extends along the ridge, 

 limiting the body of the shell from the anterior alation. In the cast of the 

 left valve, the umbonal cavity is marked by numerous nodes indicating pits 

 in the shell for muscular attachment. The posterior muscular impression is 

 large, occupying the post-umbonal slope at a point about half-way from 

 the beak to the base of the shell. 



A cast of the interior has a length of about 78 mm., height 77 mm., and 

 hinge-line less than the length of the shell. 



