stant slight involuntary action. This being the case, I conceive
that the small, blunt, spur-like portion of the peduncle, descending
beneath the basal rim of the lowest disc, would inevitably partake
slightly of the movements of the whole distended animal. As soon as the
Lithotrya has reached that depth, which its instinct points out as most
suitable to its habits, the discs are converted into an irregularly
growing cup, and the animal then only increases in diameter, enlarging
its cavity by the action of the serrated scales on the peduncle, and of
the serrated lower edges of the valves of the capitulum. With respect to
those reversed individuals attached with their capitulums downwards, I
suppose that the larvae had crept into some deep cavity, perhaps made
originally by a Lithotrya, of which the rock in the specimen in question
was quite full, and had there attached themselves. Finally, it appears
that in Lithotrya the burrowing is simply a mechanical action; it is
effected by each layer of shell in the basal attached discs overlapping,
in a straight line, the last-formed layer,--by the membrane of the
peduncle and the valves of the capitulum having excellent and often
renewed rasping surfaces,--and, lastly, by the end of the peduncle (that
is homologically the front of the head) thus roughened, extending
beyond the surface of attachment, and possessing the power of slight
movement.

[66] Mr. Hancock, in his admirable account of his burrowing
Cirripede, _Alcippe lampas_, ('Annals of Nat. Hist.,' Nov. 1849,
p. 313,) came to this conclusion regarding the cup of Lithotrya,
and hence was led to think that this genus did not form its own
burrows, but inhabited pre-existing cavities. I am much indebted
to this gentleman, who has been so eminently successful in his
researches on the boring powers of marine animals, for giving me
his opinion on several points connected with the present
discussion.

[67] I owe to the great kindness of Prof. Steenstrup the sight of
this Plate, published in the 'Scientific Communications from the
Union of Natural History,' Copenhagen, January 30, 1850, No. I.
Since this sheet has been set up in type, I have received from
Prof. Steenstrup the memoir, in Danish, belonging to the figures
in question; and the greater part of this has been translated to
me by the kindness of a friend. My account of the means of
burrowing is essentially the same as