_Shell purplish-brown, with the parietes internally strongly ribbed
and reticulated: carino-lateral compartments extremely narrow, not
extending down to the basis._
_Hab._--Sooloo Islands, East Indian Archipelago; Mus. Dana.
I am indebted to Mr. Dana, the distinguished naturalist of the United
States Antarctic Expedition, for two specimens of this interesting
species, which, in the singular reticulated structure of the inner
surface of the walls, and in the almost rudimentary condition of the
carino-lateral compartments, not extending down to the basal cup, is
very distinct from the foregoing species. I have used Mr. Dana's very
appropriate MS. name of _sporillus_. The specimens were dredged up,
lying quite loose and unattached at the bottom of the Sooloo sea; the
one which I opened, must have long lain dead; but Mr. Dana assures me
that some were living, and he has sent me drawings of parts of the
mouth and cirri: I am much surprised at this circumstance; for analogy
would have made me believe that this species must have been imbedded
in some sponge-like body, such as the bark of a zoophyte, and that it
could not have lived unattached. I may add that a small fragment of a
brown leathery substance adhered to the upper end of one of the two
specimens, and this seems to indicate attachment.
_General Appearance._--Shell shaped like a pointed acorn; slightly
flattened; orifice extremely small; surface very finely punctured,
covered by a purplish-brown epidermis, with transverse stripes of
different shades, and with the apex dark; according to Mr. Dana, when
fresh, the colour was purplish-carmine. Radii narrow, white. The
carino-lateral compartments are extremely narrow; the wall-portion
(fig. 9 _b_) forming a mere linear rib, terminating downwards in
a sharp point, which does not reach the basal cup: hence this
compartment evidently tends to become rudimentary. The basal cup is
moderately deep and pointed. Basal diameter .16; height, from the
bottom of the cup to the top of the shell, .24 of an inch.
_Scuta_: narrow, with the upper part produced; not striated
longitudinally; coloured by a pale purple, longitudinal band.
Internally, there is scarcely a trace of an articular ridge, which,
in the other species, is always more or less developed. _Terga_ with
the spur bluntly pointed; nearly the whole basal margin, on the
carinal side, slopes towards the spur.
_Internal Structure of the Parietes, Radii, and Basal Cup._--The
parietes are strongly ribbed internally; and these ribs are connected
by very narrow, less prominent, transverse, slightly branched ridges,
giving a reticulated structure to the inner surface. Between several
of the main longitudinal ribs, in the lower part of the shell, new
ribs may be seen in process of formation, and these tend to convert
the reticulated structure into a double row of minute cells. I have
not met with an exactly similar structure in any other cirripede; but
I have no doubt that the little transverse ridges are homologous with
the transverse calcareous septa in the parietal pores of many Balani,
in the same manner as the internal longitudinal ribs, in this and
other species of Acasta, are homologous with the longitudinal septa
forming the above pores. The edge of the basal cup is pectinated with
teeth, which lock into the teeth formed by the ends of the internal
parietal ribs. The radii are narrow, and have smooth edges. The alæ
project beyond the parietes to a remarkably small extent. The sheath
is free, or hollow beneath. I have already described the almost
rudimentary condition of the carino-lateral compartments; this is
best exhibited in an internal view of the two compartments, as given
in Pl. 9, fig. 9 _b_.
_Animal's body_ unknown to me: from Mr. Dana's drawing the three
posterior pairs of cirri seem to have been much elongated: and the
rami of the first pair, as usual, unequal in length.
M. Deshayes has given an indifferent figure and imperfect description
of ACASTA TUBULOSA (Guerin, Magasin de Zoologie, 1831, Tab. 39; and
Guerin, Iconographie du Règne Animal, Mollusques, Tab. 38, fig. 4, but
here by a misprint called _A. spinulosa_); it is utterly impossible to
recognise the species of this genus from such materials.