BORERS. 307 
that of Pholadidea, have been chemically examined 
by our friend, Mr. Trenham Reeks, with a nega- 
tive result as regards the presence of particles of 
silex in their substance, where, after the statement 
of Mr. Hancock respecting the structure of the 
mantle, we thought they might possibly be found. 
On the other hand, taking into consideration its 
mineralogical nature, as stated by M. Necker, there 
is no reason for supposing that the shell of the Pho- 
ladide is so weak a perforating instrument as some 
have fancied. With its peculiar form, and the 
saw-like asperities of its surface, especially of its 
anteal extremity, it is well adapted for an auger, 
when wielded fresh and elastic by its well-muscled 
animal inhabitant, whose foot, in all the members 
of this tribe, even in Zeredo, where it is least de- 
veloped, seems specially organized to serve as a 
fulcrum. We have no evidence that they perforate 
any substances essentially harder than their shells, 
or so hard. ‘The sandstones in which they occa- 
sionally occur are either friable or marly, when 
fresh, though cabinet specimens seem so solid. 
The explanation of Necker accounts for their per- 
forations in the hardest limestones. Wood, wax, 
and other substances in which they occur, offer no 
difficulty. The statements put forward respecting 
their boring in lava and granite, have long ago 
been shown to be mistakes. That they exhibit a 
rotatory motion, during the action of boring, has 
been proved by competent observers; and the 
cavities they excavate, if examined when fresh, 
invariably show transverse groovings, which could 
have been caused only by such motions. Currents 
of water, set in motion by cilia, doubtless aid mate- 
rially the animal’s operations, and possibly may 
