The holes, moreover, are not quite
cylindrical, but broadly oval, like the section of the animal. The
simple fact, that in this genus alone each fresh shelly layer round the
bases of the valves, and therefore at the widest part of the capitulum,
are sharply toothed; and secondly, that in this genus alone a succession
of sharply serrated scales, on the upper and widest part of the
peduncle, are periodically formed at each exuviation; and that
consequently the teeth on the valves and scales are sharp, and fit for
wearing soft stone, at that very period when the animal has to increase
in size, would alone render the view probable that the Lithotrya makes
or at least enlarges the cavities in which it is imbedded.

Although it may be admitted that Lithotrya has the power of enlarging
its cavity, how does it first bore down into the rock? It is quite
certain that the basal cup is absolutely fixed, and that neither in form
nor state of surface it is at all fitted for boring.[66] I was quite
unable to answer the foregoing question, until seeing the admirable
figures by Reinhardt[67], (Pl. VIII, figs. 2 and 2 _a'_) of _L.
Nicobarica_, still attached in its cavity. Subsequently I obtained from
Mr. Stutchbury several pieces of rock completely drilled with holes,
many of small diameter, by _L. dorsalis_, and in these I found numerous
instances of the linear rows of little discs, like those of _L.
Nicobarica_, showing in the plainest manner, that each time a new disc
is formed, that is, at each exuviation, the animal moves a short step
downwards; and as the lowest of these little discs _in none of the
burrows_ was placed at the very bottom, we see that the lowest point of
the peduncle must be the wearing agent. In the peduncle of an
individual of _L. dorsalis_, nearly ready to moult, I found, it may be
remembered, beneath and round the basal disc, under the old membrane of
the peduncle, a new membrane studded with calcified beads, but with the
horny star-headed spines not yet developed, whilst on the old outer coat
these latter had been worn down quite smooth, and the calcified beads
worn entirely away. Here, then, we have an excellent rasping surface.
With respect to the power of movement necessary for the boring action,
the peduncle is amply furnished with transverse, oblique, and
longitudinal striae-less muscles,--the latter attached to the basal disc.
In all the pedunculata, I have reason to believe that these muscles are
in con