ON ACEPHALA. 407 
they actually feed upon them. Lithophagous species are 
found in almost all the families of the bivalves. The majority 
live in our European seas, and especially in the waters 
of the Mediterranean; yet, notwithstanding the facility of 
observation, we are still ignorant of the process pursued by 
these lithophagous animals, to penetrate thus into the interior 
of the stones. Some persons have thought that it was only 
when the stones were soft that this could be done, because, in 
fact, the pholades are found in a sort of white soft argile, 
which has been regarded as a sort of incipient stone. But 
they are also found in the true lime-stone, more yielding and 
softer, no doubt under the water, than when it is exposed to 
the air. This opinion was supported by Reaumur, and by 
Lafaille, of the Academy of Rochelle. M. Fleurian de Belle- 
vue, who made his observations in the same places as the 
last mentioned naturalist, was convinced that the pholades, 
however small they may be, pierce the calcareous stone itself; 
and M. de Blainville has seen, on the coasts of Normandy, 
the same species of pholas in the clayey depositions of the 
mouth of the Seine, and in the tolerably hard calcareous mass 
of the chalk formation, which borders the sea for a great 
portion of its extent. Moreover, pholades and lithophagous 
mussels are sometimes found in marble on the coasts of the 
Mediterranean. ‘The direction which the lithophagous mol- 
lusca take, in the substance where they conceal themselves, 
varies according to the genera. The pholades place them- 
selves vertically ; but not so the Saxicava, and some approxi- 
mating genera ; these animals pierce the stone in all directions, 
so as sometimes to meet one another. If we should admit 
that the pholades, whose shell is pretty thick, and furnished 
with asperities at its anterior extremity, can excavate their 
stony lodge, by mechanical means, in turning on themselves, 
the thing is conceivable enough, because they are free. But 
this can hardly be the case with others, which fill the cavity 
