always considerably larger than the
other valves: in the different genera the valves differ so much in shape
that little can be predicated of them in common; even the direction of
their lines of growth differs,--thus, in Lepas and some allied genera,
the chief growth of the scuta and of the carina is upwards, whereas in
Pollicipes and Lithotrya, it is entirely downwards; in Oxynaspis, and
some species of Scalpellum, it is both upwards and downwards. Even in
the same species, there is often very considerable variation in the
exact shape of the valves, more especially of the terga. The adductor
muscle is always attached to a point not far from the middle of the
scuta, and it generally has a pit for its attachment. In several genera,
namely, Paecilasma, Dichelaspis, Conchoderma, and Alepas, the scuta show
a tendency to be bilobed or trilobed. The valves are placed either at
some distance from each other, or close together; but their growing
margins very rarely overlap each other, though this is sometimes the
case with their upper, free, tile-like apices; in a few species the
scuta and terga are articulated together, or united by a fold. The
membrane connecting the valves, where they do not touch each other, is
like that forming the peduncle, and is sometimes brilliantly coloured
crimson-red; generally, it appears blueish-gray, from the corium being
seen through. Small pointed spines, connected with the underlying
corium by tubuli, are not unfrequently articulated on this membrane: the
tubuli, however, are often present where there are no spines. To allow
of the growth of the capitulum, the membrane between the valves splits
at each period of exuviation, when a new strip of membrane is formed
beneath, connected on each side with a fresh layer of shell,--the old
and outer slips of membrane disintegrating and disappearing: when there
are many valves, the line of splitting is singularly complicated. This
membrane consists of chitine,[13] and is composed of numerous fine
laminae. After the valves have been placed in acid, a residue, very
different in bulk in different genera, is left, also composed of
successive laminae of chitine. It appears to me that each single lamina
of calcified chitine, composing the shell, must once have been
continuous with a non-calcified lamina in the membrane connecting the
several valves: at the line where this change in calcification
supervenes, the chitine generally assumes some colour, and become