91 
tus; but it agrees very well, and is probably specifically the same, 
with the species represented on plate 365 of Lister’s Conch., which 
the author thus defines, musculus parvus, suhluteus, leviter stria- 
tus.” 
Mytillus lateralis. — Shell transversely suboval, inflated, 
subpellucid, with numerous concentric wrinkles, anterior and pos- 
terior margins, longitudinally ribbed with alternate large and small 
lines, which crenate the basal margin ] intermediate area destitute 
of longitudinal lines ; most prominent part of the shell extending 
from the beak to the tip of the anterior margin, and bounded on 
its posterior side by an indented line ; epidermis pale-brownish. 
Length three-tenths of an inch. Breadth eleven-twentieths of 
an inch. Thickness seven-twentieths of an inch. Inhabits the 
southern coast. Cabinet of the Academy and Philadelphia Mu- 
seum. 
Found imbedded in the large Thethya of our coast. This shell 
is closely allied to Mytillus discors of Montague. 
Mytillus hamatus. — Shell very much contracted and incurved 
at the base, which is acute ; valves striated on every part of the 
exterior with longitudinal, elevated lines, which are bifid and some- 
times trifid towards the tip ; color dark fuscous ; within dark pur- 
purescent with a whitish margin. 
Length one and one-fifth of an inch. Breadth nearly four-fifths 
of an inch. Inhabits the Gulf of Mexico. Cabinet of the Acade- 
my and Philadelphia Museum. 
A common species in the Gulf of Mexico, and is frequently car- 
ried to market at New Orleans, attached to the common oyster. It 
is remarkably distinct from M. demissus by the great incurvature 
of the beaks, by its inferiority in magnitude, and by having the 
striae extending over every part of the surface of the shell. 
I cannot refer it to strialulus, with which it seems to have some 
affinity, as that species is described to be nearly diaphanous. It 
seems to have some affinity with the M. decussatus, Lam,, but is 
much smaller, and the transverse striae are not very prominent. 
Modiola Americana. — Yar. Shell oblong, hinge margin ele- 
vated in a right line from the beak to the alated angle, from which 
it declines also in a right line nearly to an equal distance ; alated 
projection rounded j anterior margin short and small ; basal margin 
