141 
PETRICOLA. Lamarck. 
Gen. Char. An unequal-sided,, equal-valved, trans- 
verse bivalve ; the anterior side smallest, round- 
ed, the posterior produced, rather gaping; 
teeth of the hinge three or four, small, curved ; 
no lateral teeth ; a large sinus in the mark of 
the attachment of the mantle ; ligament ex- 
ternal, short. 
Under the generic name Petricola are arranged some 
of the shells belonging to animals that have the power 
of forming hollows in calcareous rocks, not by a rotatory 
motion as the Pholas does, but by some solvent power 
that enables them to make holes that nearly fit the shells. 
Dirt and foreign substances that are not calcareous often 
make way into these holes ; and as they cannot easily be 
removed, they interfere with the growth of the shells, 
which are consequently often distorted*. 
The species are almost as variable in their external 
forms as in the number and form of their teeth ; some 
are nearly smooth, others striated and even rough like 
Pholades, some depressed, others gibbose ; they are 
mostly white, but one or two are coloured. They are 
all tender shells. 
* Lithodomi and some other boring shells belong to animals pos- 
sessed of the same power, and which at the same time line a por- 
tion of the hole with a calcareous deposit. The animals of many 
spiral shells operate in a similar way upon their own shells, re- 
moving parts of their surface or spines to lighten them or make 
room for their further growth. In these cases, does the animal 
secrete two kinds of fluid, or, which seems more probable, is it 
endowed with the faculty of changing the electric state of the 
same fluid, so as to make it dissolve or deposit the lime at differ- 
ent seasons or as occasion may require ? 
