154 Couthouy's Monograph on 
shell. Ligament, externally, very prominent, and pro- 
longed in a thin membrane the whole length of the 
corselet, which is strongly marked and extends from the 
beaks to the extremity ; the internal portion of the liga- 
ment is attached to a strong, thick, nymphal callosity, 
projecting obliquely along the cardinal edge in each 
valve, wider toward the beaks, and having its surface 
but very slightly hollowed. Hinge destitute of a car- ` 
dinal ossiculum. External color a pale ashy-white, 
surface covered with a thin, light cinereous epidermis, 
strongly adherent and forming numerous irregular, mi- 
nute corrugations at the extremities, especially on the 
posterior one, but not shagreened as in T. corbuloides. 
Internal color a chalky white, not glossy, but somewhat 
inclining to nacre near the cardinal edge, Muscular 
impressions tolerably large, remote, the anterior narrow, . 
elongated, contracted and tapering to a point towards . 
the hinge margin; the posterior sub-triangular or pyri- 
form ; palleal impression very superficial like the others, 
with a profound, sub-angular excavation posteriorly, 
- Length two and eighteen-twentieths, height two and 
four-twentieths, diameter one and'‘six-twentieths inches. 
Inhabits probably the whole coast of New England. 
Osservations. Single valves of this shell are occa- 
sionally met with, along the whole coast of New Eng- 
land, but. perfect specimens are extremely rare. - The 
beach at Chelsea is the only locality, within my know- 
ledge, where such have béen found. . Of late years sev- 
eral have been procured at this place, shortly after the 
breaking up of the ice; cast up by the heavy sea which 
during the violent wes zales of winter, rolls in upon 
