60 Couthouy's New Species of Mollusca 
over each other in faint concentric laminations; anterior 
extremity very regularly rounded; posterior slightly 
angular, with the characteristic plait well marked ; right 
valve with two cardinal teeth, the anterior one simple, 
the posterior bifid, or rather bi-cuspidate; left valve 
having them almost always obsolete ; in a few cases a 
single slender posterior tooth is found, entering into the 
bi-cuspidate one opposite; a feeble compression and 
elevation of the posterior cardinal plates forms a remote, 
inconspicuous lateral tooth in each valve. Internal color 
an uniform milk-white, inclining to iridescence. 
Osservations. Several specimens of this shell were 
taken while fishing about two miles outside of Boston 
lighthouse. Its general aspect approaches that of San- 
guinolària fusca, Conrad, but it is easily distinguished, 
by being less convex and rounded and more inequilateral, 
and by the fold upon the posterior portion. The internal 
color is moreover always white instead of yellowish or 
reddish, as in that shell. 
It cannot be T. alternidentata, Brod. & Sow., from 
the Arctic Seas, though their description is applicable in 
some points, as that is nearly two and a half inches in 
length and has two alternating cardinal teeth only in 
each valve. T. inconspicua, same authors, seems to 
approximate more closely, but has no lateral teeth (obso- 
lete perhaps); the number of cardinal teeth is not stated. 
It would seem to be a much more convex shell than 
ours, its diameter being eight twentieths of an inch, or 
more than half its height, fifteen twentieths. From this 
circumstance, and their comparison of it to T. solidula, 
Solan., it is doubtful whether their T. inconspicua is not 
identical with S. fusca, Conr., Ps. fusca, Say, of our 
own shores. A good figure would decide this question. 
