_Scutum much elongated, with a tooth-like articular projection: tergum
convex, irregularly triangular, sometimes with an imperfect spur, and
on the internal surface with an inwardly projecting tooth; scutum and
tergum furnished with an occludent ledge._
_Var._ (1), 3 _c_, 3 _g_: _tergum, with a sharp internal tooth,
projecting rectangularly inwards_.
_Var._ (2), 3 _d_: _tergum, with a broad blunt internal tooth,
depending beneath the spur-like portion of the valve_.
_Var._ (3), 3 _f_: _tergum, with the basi-carinal end of the valve
truncated, with a small blunt internal tooth projecting rectangularly
inwards_.
_Hab._--Red Sea; Mus. Brit. and Cuming. Also associated with _Pyrgoma
crenatum_, and attached to _Meandrina spongiosa_.[120]
[120] Mr. Dana informs me that he believes that this coral comes
from the West Indies; though the specimens originally described
by him had no label. If this be so, both _Pyrgoma dentatum_ and
_crenatum_ have very wide ranges.
_Appearance and Structure of Shell._--Shell nearly flat, oval,
white or pink, with rather distant prominent ridges radiating from
the moderately large (for the genus) oval orifice. The ridges are
often obscured, and apparently sometimes almost obliterated by the
encrusting coral. Shell permeated near the outer lamina by short
imperfect pores: internal surface smooth: sheath inconspicuous,
descending rather more than half way down the walls; lower edge
closely attached to the walls. Basis deep. Diameter of largest
specimen .3 of an inch.
_Scuta_: these are elongated, but to a very variable degree, some
specimens being quite three times, and some barely twice as broad
as high. I observed this same variability in two sets of specimens,
differing, as we shall presently see, in the form of their terga:
it depends in part, but not wholly, on the varying width of the
occludent ledge, which is sometimes only a fourth, and sometimes
half as high as the rest of the valve. This is the first species
in the genus in which the occludent ledge--a structure peculiar
to the genus--has been amply developed. The basal margin of the
valve is slightly sinuous, and a very little hollowed out near the
basi-tergal corner; it is also very slightly reflexed, the reflexed
portion being separated from the upper part of the valve by a very
slight depression or even furrow. I notice this slightly reflexed
portion, simply as indicating a well marked feature in the basal
margin of the following closely allied species. Internally (3 _g_),
the adductor ridge is thick and slightly prominent, but does not
descend beneath the basal margin: it blends into the articular ridge,
which here projects in a remarkable manner and degree (3 _b_, 3 _e_,
3 _g_), like a rounded tooth. This tooth is in part a development of
the occludent ledge; it varies much in shape. The line of junction
between the scutum and tergum is nearly straight, and nearly at right
angles to their longer axes. In some specimens the scuta and terga
are partially calcified together.
_Terga_: in three sets of specimens the terga differed considerably,
but as in every other respect there was the closest resemblance,
I do not doubt that these are merely varieties. In all three, the
valve is rather small, irregularly sub-triangular in shape, and
externally somewhat convex; in all three, there is an occludent
ledge, of variable width as in the scuta; and in all three, there is
an internal tooth-like projection, of variable form, unlike anything
I have seen in any other cirripede. In the _first variety_ (Pl. 13,
fig. 3 _c_, 3 _g_), the basi-carinal corner of the valve is bluntly
pointed, and a slight flexure separates this portion of the valve
from the other and scutal half, which latter thus exhibits some
tendency to be converted into a spur: on the internal surface (3
_g_) of this spur-like portion of the valve, there is a rather long,
sharp tooth, which projects rectangularly inwards; it is flattened
in a plane at right angles to the longer axis of the scutum and
tergum together: it cannot be seen from the outside. In the _second
variety_, the shape of the valve is not very different (Pl. 13, fig.
3_d_), excepting that the flexure, separating the basi-carinal corner
of the valve from the spur, is deeper; but on the internal face of
the spur, the tooth is far broader than in the first variety, and
is flattened quite differently, viz., in a plane nearly parallel to
the surface of the valve, and instead of projecting rectangularly
inwards, it depends beneath the basal edge of the so-called spur, and
can be seen from the outside. In the _third variety_ (fig. 3 _f_),
the whole carinal end of the valve is cut off, and there can hardly
be said to be any trace of a spur, yet a slight furrow apparently
marks the line of separation between the basi-carinal portion of
the valve, here become very narrow, and the broad, irregular part,
which would have formed the spur had such been developed: on the
internal surface of the latter portion of the valve, there is a very
small, blunt, slightly flattened tooth, projecting inwards, and more
resembling that in the first than that in the second variety.
_Affinities_: under _P. crenatum_ I will point out the diagnosis and
relationship between this and that species.