3@3 
and boring marine animals. 
a third of its proper thickness ; yet neither of the three 
showed the slightest mark of friction, and the cuticle of the 
sides in contact was as perfect as usual. 
But independent of the presumption afforded by the soft- 
ness and smoothness of the shell, and by the absence of any 
arrangement of muscles which might employ it with effect 
had it been strong enough to act on hard stone, it is easy to 
collect facts affording ample proof that the shell cannot be 
the instrument of penetration. I have a specimen of calca- 
reous stone of extraordinary hardness, containing small 
masses of silex, some of which project into the holes formed 
by Saxicava rugosa, and Venerupis irus. The lime has been 
smoothly cleared around the base of these projecting portions 
from situations with which the shell could not possibly have 
come into contact. In another specimen, in which the lime 
is mixed with a large proportion of clay, there is a small 
round stratum of stone entirely argillaceous, and not one- 
twentieth of an inch in thickness. The lime surrounding this 
stratum has been removed by three Saxicavas, which have 
bored down upon it from different directions. The shell of 
one of them remains in its hole, and shows that the pro- 
gress of the animal was arrested by the stratum long before 
its death. There is a deep groove across one valve, in which 
the stratum is nearly buried ; and which has evidently been 
caused, not by friction against the stratum, but by the growth 
of the shell on each side of it ; this is shown in fig. 6. 
If additional proof is required, it is supplied by the fact that 
many naked animals of the softest texture form their habi- 
tations in limestone. The boring Annelides are innumerable 
in calcareous rocks, and are found to attack every marine 
