valves in the capitulum has in this genus
acquired its maximum. The number varies considerably in the same
species, and even on opposite sides of the same individual, and
generally increases with age. It is more important, that the number of
the whorls in _P. cornucopia_, and in the two following closely-allied
forms, also increases with age. In _P. sertus_ and _P. spinosus_, even
the number of the whorls varies in different individuals, independently
of age. The valves are arranged alternately with those above and below;
they are generally thick and strong, making the capitulum somewhat
massive; in some species they are subject to much disintegration; but in
others, the apices of the several valves, especially of the carina and
rostrum, are well preserved, and project freely: they are covered with
membrane, which, differently from in most species of Scalpellum, either
does not bear any spines, or only exceedingly minute points. In all the
species there is a sub-rostrum and sub-carina, and often beneath these a
second sub-rostrum and sub-carina. In medium-sized specimens there are
at least 20 valves in the lowermost whorl. The carina is either straight
or curved, but never rectangularly bent, and is always of considerable
breadth. None of the valves are added to at their upper ends. The scuta
have a deep pit for the adductor muscle. The valves lie either some
little way apart, or more commonly close together. In _P. mitella_ the
scuta and terga are locked together by a fold, and the valves of the
lower whorl overlap each other in a peculiar manner, resembling that in
which the compartments in the shells of Sessile Cirripedes fold over
each other.

The _Peduncle_ is of considerable length in some of the species, and
rather short in others; it is, in every case, clothed with calcified
scales. The scales in the first four species are placed alternately and
symmetrically; they are formed and added to in the same manner as in
Scalpellum; they differ in size according to the size of the individual,
and consequently the lower scales on the peduncle, formed when the
specimen was young, are smaller than the upper scales; the lower scales
are separated from each other by wide interspaces of membrane, owing to
the continued growth of the peduncle by the formation of new layers of
membrane, and the disintegration of the old outer layers. Each scale is
invested by tough membrane (or has been, for it is often abraded off),
in the sam