this period. I may here
add, that the pedunculated Cirripedes never attain so large a bulk as
the sessile; _Lepas anatifera_ is sometimes sixteen inches in length,
but of this, the far greater portion consists of the peduncle.
_Pollicipes mitella_ is the most massive kind; I have seen a specimen
with a capitulum 2.3 of an inch in width.

_Affinities._--Considering the close affinity between the several
genera, there are, I conceive, no grounds for dividing the Lepadidae into
sub-families, as has been proposed by some authors, who have trusted
exclusively to external characters. In establishing the eleven genera in
the Lepadidae, no one part or set of organs affords sufficient diagnostic
characters: the number of the valves is the most obvious, and one of the
most useful characters, but it fails when the valves are nearly
rudimentary, and when they are numerous: the direction of their lines of
growth is more important, and fails to be characteristic only in
Scalpellum: with the same exception, the presence or abscence of
calcified or horny scales on the peduncle is a good generic character.
For this same end, the shape of the scuta and carina, but not of the
other valves, comes into play. In three genera, the presence of
filamentary appendages on the animal's body is generic; in Pollicipes,
however, they are found only on three out of the six species. The number
of teeth in the mandibles, and the shape of the maxillae, often prove
serviceable for this end; as does more generally the presence of caudal
appendages, and whether they be naked or spinose, uniarticulate or
multiarticulate; in Pollicipes alone this part is variable, being
uni-and multi-articulate; and in one species of Scalpellum they are
absent, though present in all the others. The shape of the body, the
absence or presence of teeth on the labrum, the inner edge of the outer
maxillae being notched or straight, the prominence of the olfactory
orifices, the arrangement of the spines on the cirri, and the number and
form of their segments, are only of specific value.

Comparing the pedunculated and sessile Cirripedes, it is, I think,
impossible to assign them a higher rank than that of Families. The chief
difference between them consists, in the Lepadidae, in the presence of
three layers of striae-less muscles, longitudinal, transverse and
oblique, continuously surrounding the peduncle, but not specially
attached to the scuta and terga; and on the other hand, i