t margin is highly protuberant and arched, or even formed of two
distinct sides.

Occasionally, I have referred to what I have called the _primordial
valves_: these are not calcified; they are formed at the first
exuviation, when the larval integuments are shed: in mature Cirripedes
they are always seated, when not worn away, on the umbones of the
valves.

The membrane connecting the valves, and forming the peduncle, and
sometimes in a harder condition replacing the valves, I have often found
it convenient to designate by its proper chemical name of _Chitine_,
instead of by horny, or other such equivalents. When this membrane at
any articulation sends in rigid projections or crests, for the
attachment of muscles or any other purpose, I call them, after Audouin,
_apodemes_. For the underlying true skin, I use the term _corium_.

The animal's body is included within the capitulum, within what I call
the _sack_ (see Pl. IV, figs. 2 and 8' _a_, and Pl. IX, fig. 4). The
body consists of the _thorax_ supporting the cirri, and of an especial
enlargement, or downward prolongation of the thorax, which includes the
stomach, and which I have called the _prosoma_. (Pl. IX, fig. 4 _n_).
The cirri are composed of two arms or _rami_, supported on a common
segment or support, which I call the _pedicel_. The _caudal appendages_
are two little projections, either uni-or multi-articulate (Pl. IV, fig.
8' _a_), on each side of the anus, and just above the long
proboscis-like penis. On the thorax and prosoma, or on the pedicels of
the cirri, there are in several genera, long, thin, tapering filaments,
which have generally been supposed to serve as branchiae; these I call
simply _filaments_, or _filamentary appendages_ (Pl. IX, fig. 4 _g-l_).
The mouth (fig. 4 _b_) is prominent, and consists of _palpi_ soldered to
the _labrum_; _mandibles_, _maxillae_, and _outer maxillae_, these latter
serve as an under lip; to these several organs I sometimes apply the
title used by Entomologists, of "trophi." Beneath the outer maxillae,
there are either two simple orifices or tubular projections; these, I
believe, serve as organs of smell, and have hence called them the
_olfactory orifices_. Within the sack, there are often two sheets of ova
(Pl. IV, fig. 2 _b_), these I call (after Steenstrup, and other
authors) the _ovigerous Lamellae_; they are united to two little folds of
skin (Pl. IV, fig. 2 _f_), which I call the _ovigerous Fraena_.

From the p