rutilum_ are considerably nearer to
each other than any other two of the species. Upon the whole I conclude
that the six species must be thrown either into five or into four genera
(the first three species making one genus), or all into one genus, and
this latter has appeared to me the preferable course. The separation
even of Scalpellum and Pollicipes, as already stated, is hardly natural.
The fact of these genera having existed from a remote epoch, and having
given rise during successive periods to many species now extinct, is
probably the cause that the few remaining species are so much more
distinct from each other, than is common in the other genera of
Lepadidae. Whenever the structure of the whole capitulum in the fossil
species is well known, and as soon as more species, recent and fossil,
shall have been discovered, then probably the genus Scalpellum will have
to be divided into several smaller genera.

_Description._--The _Capitulum_ is much compressed, and generally
produced upwards; it is formed of from twelve to fifteen valves, which
are rather thin, and with the exception of _S. ornatum_, almost entirely
covered by membrane, bearing spines: the valves are seldom locked very
closely together. A sub-rostrum exists only in _S. villosum_, which
species leads on to Pollicipes; in _S. vulgare_ the rostrum is
rudimentary and hidden. The scuta, terga and carina, are much larger
than the other valves: these five valves seem to differ essentially
from the others in being at first developed under the form of the
so-called primordial valves: the other valves commence by a small
indistinct brown spot, very different from the hexagonal tissue of the
primordial valves: I saw this very clearly in young specimens of _S.
vulgare_. At first, the scuta, terga and carina, grow exclusively
downwards (and permanently so in most fossil species), and therefore the
growth of the scuta and carina is in an absolutely opposite direction to
what it is in Lepas, Paecilasma and Dichelaspis. After a short period the
scuta are added to at their upper ends; the portion thus added, stands
at a rather lower level, and projects in a rather different direction
from the first-formed part of the valve, giving to it, in some respects,
the appearance of having been broken and mended. This structure is
common to _S. vulgare_, _S. rostratum_ and _S. Peronii_. The upper
Latera (except in _S. villosum_) grow in the same manner, namely, at
first exclusiv