90 
margin of one valve^ closes over tliat of tlie other. This species 
is very different from the P. insequivalvis, particularly in having 
the hinge placed much further hack, and consisting of a mere 
angle, not prominent ; the rostrum also has a direction more up- 
ward. 
Avicula hirxjdo. — Var. Shell perlaceous; epidermis reddish- 
brown, with very numerous undulated wrinkles, which are disposed 
ill radii, and rendered more conspicuous by a white longitudinal 
line at the junction of each series of wrinkles with its contiguous 
one. 
Width about three-fourths of an inch. Inhabits the southern 
coast. Cabinet of the Academy and Philadelphia Museum. 
It appears to be rare. I have found but a single entire speci- 
men, which is young. In its radiating series of wrinkles, it ap- 
proximates to A. morio of Leach, but differs from it in magnitude, 
and in being radiated with whitish lines. I have a specimen from 
the West Indies, which corresponds very well with this, but as it 
is a much older shell, it is of a much darker color, and the radii 
are interrupted into abbreviated lines. 
A valve of an adult shell also occurred on the southern coast, 
but so much worn by attrition that its superficial characters are 
destroyed. 
Mytillus cubitus.— Shell oblong, striated with elevated, sub- 
glabrous lines, which are smaller on the anterior side; anterior 
edge linear, or slightly concave ; posterior edge ascending from the 
base in a right line to a prominent angle, which is rather behind 
the middle of the shell, from which it descends by a concave line 
, to the obliquely and very obtusely rounded tip ; color yellowish, 
polished and somewhat fa.;ciated with green or brownish, which 
disappear on the anterior margin. 
Length one and one-fifth of an inch. Breadth half an inch. 
Inhabits the coast of the United States. Cabinet of the Academy 
and Philadelphia Museum. 
This species, seems to be most closely allied to M. demissus and 
exustm; from the former it is distinguished, by not having the 
angle on the posterior side obtusely rounded, and not placed con- 
siderably before the middle ; and the line of the edge before this 
angle, is not convex as in that shell. It does not at all correspond 
with the figures in the Encyc. Method, which are quoted for exus- 
