ACEPHALA. 437 
tricose, divided into two areas by a prominent but obtuscly angular 
umbonal slope; the anterior area nearly plane, the posterior convex ; 
beaks inconspicuous at the anterior fifth, much eroded; stages of 
growth loose and laminated, with some indistinct radiating strie on 
the disk ; epidermis coarse, piceous. Nacre bluish-white, with salmon 
stains in the cavity of the beaks; left cardinal margin very delicate, 
and nearly destitute of a tooth, thickened posteriorly, while the right 
valve has a conspicuous thickening at the beak, which generally rises 
into a tubercular, tooth-like apophysis. 
Length two and a quarter inches; height an inch and a quarter ; 
width three-quarters of an inch. 
Inhabits Wallawalla, Columbia River, Oregon. Dr. Pickering. 
At first this was supposed to be the young of A. angulata, Lea; but 
it proves different in several constant characters, besides its widely 
remote habitat. It is much smaller and thinner, its surface rude and 
dusky, instead of smooth and green; its outlines are all curves instead 
of right lines; the dorsal and terminal outlines form a continuous 
curve instead of a distinct angle; the posterior umbonal slope is ob- 
tusely angular instead of having the remarkably acute carination of A. 
angulata, and the area posterior to it is convex and not concave; the 
beaks are more anterior, and the cardinal apophysis is never found in 
A. angulata, while it is constant in this. 
Figures 547, 547 a, 547 d, three views of the shell. 
AvicuLa MaRGARITIFERA (Liy.), Roissy. Meleagrina margaritifera, 
Lamarck; Anim. sans Vert., vil. 107. 
Suet rounded-trapezoidal, compressed, attaining great size and 
weight. Colour sea-green, radiated with whitish stripes; surface in 
distant loose lamine, which in young specimens project into from 
fifteen to twenty series of long, spatulate scales corresponding to the 
pale stripes. Hinge margin long and straight, the posterior* margin 
passing off at nearly a right angle without forming a queue, and after 
passing half the length in a right line, gently curves around the ventral 
* The side from which the byssus issues, is considered the anterior. 
110 
