Position of the animal in the rock, and its power of excavation._--A
specimen of rock, two or three inches square, in Mr. Cuming's
possession, is full of Lithotryas; the cavities extend in every possible
direction, and several were parallel, but with the animals in reversed
positions; the same thing is apparent in some specimens of Mr.
Stutchbury's, and it was evident that the positions occupied by the
animals were entirely due to chance. In Mr. Cuming's specimen of rock, a
considerable portion of the external surface is preserved, and here it
can be seen that many of the specimens have their capitulums directed
from the external surface directly inwards. These individuals, which
were of full size, must have preyed on infusoria inhabiting the cavities
of the porous, calcareous rock. On the other hand, I have seen some
young specimens of _L. dorsalis_ with their valves not at all rubbed,
and others of full size with uninjured Balani and corallines on the tips
of the valves, and again a specimen of _L. truncata_ with minute
pale-green sea-weed on the summit of the capitulum,--all which
appearances induce me to believe that in these cases, the valves had
projected freely beyond the cavity in which their peduncles were lodged.
I may here also mention that in Mr. Cuming's specimen, above alluded to,
the basal cups of five specimens touched and adhered to each other; I
was not able to make out whether there had originally existed separate
burrows, as I think is most probable, and that the walls had been wholly
worn away, or whether the five specimens had fixed themselves on one
side of a large pre-existing, common cavity. Young specimens seem to
burrow to the full depth, before nearly acquiring the diameter which
they ultimately attain. I measured one burrow, 1.2 of an inch in depth,
which, at its mouth or widest part, was only .17 in diameter.

The several species occur imbedded in soft calcareous rocks, in massive
corals, and in the shells of mollusca and of cirripedes. It has been
doubted by several naturalists, whether the basal calcareous cup at all
belongs to the Lithotrya, but after the foregoing microscopical
observations on its structure, it is useless to discuss this point. So
again it has been doubted whether the cavity is formed by the cirripede
itself; but there is so obvious a relation between the diameters of
specimens of various sizes, and the holes occupied by them, that I can
entertain no doubt on this head.