_Parietes solid; rostrum nearly twice as long as the carina or
carino-lateral compartments, hence the basis is oblique. Tergum with
the spur truncated, half as wide as the valve._
_Hab._--West Indies; Mus. Brit.--Jamaica, imbedded in a sponge; Mus.
Cuming.
This is a remarkable species; when first seeing it imbedded in numbers
in a sponge, I did not in the least doubt but that it was an Acasta:
on examination, however, it is found to have a membranous basis, and
therefore cannot by the definition enter into that sub-genus, to which,
however, it is very closely allied. It differs from other sessile
cirripedes very remarkably in the rostrum being nearly twice as long as
the carinal compartments, so that the basis is always very oblique, or
placed almost on one side; in this elongation of the rostrum, although
in a different direction, we are reminded of _B. calceolus_ and its
allies; and these latter we know can hardly be separated from certain
species of Acasta. Hence the position of our present species in this
section, is not natural; but I am unable to place it elsewhere, without
breaking down every definition: it should stand somewhat isolated,
on one side of a line of affinity connecting _Balanus calceolus_ and
_navicula_ with _Acasta purpurata_.
_General Appearance._--The shell is thin, fragile, smooth, and
white, but covered to a considerable extent by a brown membrane,
which on the sheath and opercular valve is of a bright tint, and
clothed with bristles. Viewed laterally, the rostrum is seen to
be considerably bowed, and from its being nearly twice as long as
the other compartments, with its lower end bluntly pointed, the
basal margin of the whole shell is rendered very oblique, forming a
slightly concave line. The lateral compartments are rather longer,
and about one third broader than the carino-lateral compartments.
The rostrum, from terminating downwards in a blunt point, instead
of being square or truncated, as in all other Cirripedes, and from
the upper end being, as is usual, pointed, has, when disarticulated
from the other compartments, the shape of a boat. The parietes are
not at all porose: their internal surface sometimes shows traces
of longitudinal ribs, but sometimes there are none. The radii are
narrow, with their summits very oblique, and their sutural edges
smooth. The sutural edges of the alæ are likewise smooth. The largest
of Mr. Cuming's specimens was .2 of an inch in diameter; but a
disarticulated specimen in the British Museum must have been larger,
having a rostrum .3 in length. The _Basis_ is membranous.
The _Scuta_ are rather convex; they have their lines of growth
approximate, most finely crenated, so as to be very feebly striated
longitudinally. Internally, the articular ridge is pretty well
developed, its lower edge being very oblique; there is barely an
adductor ridge: the pit for the lateral depressor muscle is deep.
The spur of the _Tergum_ is placed close to the basi-scutal angle
of the valve; it is about half as wide as the valve, with the
lower end truncated: sometimes it may be rather said to be bluntly
pointed, owing to its carinal side sloping up to the basal margin.
The articular ridge is pretty well developed. The crests for the
depressor muscles are barely discernible.
_Animal's body_ unknown.
_Section_ F.
_Parietes and radii not permeated by pores; basis sometimes permeated
by pores, sometimes not permeated, sometimes excessively thin and
hardly distinguishable._