lengtli of tlie shell ; within bluish-white ; umbo destitute of the 
slightest elevation ; anterior and posterior margins rounded; supe- 
rior and inferior margins rectilinear, parallel. 
Length seven-twentieths of an inch. Breadth nineteen-twen- 
tieths of an inch. Inhabits the southern coast. Cabinet of the 
Academy. 
Occurs sometimes cast on shore, generally in fragments, but is 
by no means a common shell. 
Saxicava distorta. — Shell thick, inequal, rugged, trans- 
versely oblong-subovate ; epidermis pale-brownish, much wrinkled ; 
umbo prominent, placed very far back ; posterior margin rounded, 
generally very short; anterior margin often truncated, with a 
prominent ridge passing from its inferior angle to the beak. 
Length about three-fifths of an inch. Width about one inch. 
Inhabits the southern coast. Cabinet of the Academy and Phila- 
delphia Museum. 
When young, it is generally more or less contracted near the 
middle of the basal margin, but this character decreases as the 
shell increases in size, until it disappears entirely in the adult 
state. It is in other respects variable in form and proportion, the 
beaks are rarely placed so far back as to be parallel with the tip of 
the posterior margin ; it much resembles Mytillus rugosus of Lin., 
but appears to be a much thicker shell. It is generally imbedded 
in our large Tliethya^ Lam., and not unfrequently intervenes 
between the substance of the TlietTiya and the sides of a large 
Ascidiay which also attaches itself to that animal. It is also 
sometimes found in a species of spongia. Pinnotheres hyssomia 
of this Journal, inhabits this shell. The young shell is furnished 
with a prominent incrassated hinge tooth, which closes into a cor- 
responding depresssion in the opposite valve ; but this tooth dis- 
appears with age. 
It is referrible to the genus Pholeohia of Leach. 
Petricola fornicata. — Shell transversely elongated, posterior 
side very short; anterior side a little gaping; hinge and basal mar- 
gins subparallel ; valves longitudinally radiated with elevated lines, 
which, anterior to that which terminates at the middle of the base, 
are alternately more or less prominent, filiform, and all posterior to 
that line are fornicated costa ; concentric wrinkles numerous, more 
remarkable on the anterior margin ; lunule ovate-acute, simply 
