_Carino-lateral parietes about one fourth of width of lateral parietes:
radii wider than the parietes: basis nearly flat, small: tergum with
the spur truncated, half as wide as valve._
_Hab._--Madeira, Mus. Lowe, and Bowerbank. West Indies, Mus.
Stutchbury. Hab. unknown, Mus. Brit.
I feel more confidence in this case, than in that of _A. sulcata_, that
we here have a distinct species, though at one time I treated it only
as a marked variety of _A. spongites_. I rely chiefly on the great
proportional width of the radii of all the compartments, and on the
width of the carino-lateral compartments, compared with the lateral
compartments, and on the general shape of the shell, which differs
considerably from that of the two previous forms: in consequence of
Acasta being attached to and imbedded in a yielding substance, such
as sponge, I believe external form to be of more value as a specific
character in this genus, than in most sessile cirripedes. I have
examined specimens taken out of the yielding _Spongia officinalis_ and
out of an unusually compact sponge, and they resembled each other in
every respect.
_General Appearance._--Colour pale pink, or that of flesh: basis
remarkably flat and rather small, with the walls above bulging out
a little. The radii are very wide, being wider than the parietes
to which they belong: the orifice is generally rather large. The
parietes of the carino-lateral compartments vary from one third to
one fourth of the width of the parietes of the lateral compartments.
Basal diameter of largest specimen .35 of an inch. Internally, the
parietes are generally more strongly ribbed than in _A. spongites_.
The _Opercular Valves_ are large, owing to the form of the shell. The
_Scuta_ present no particular character, and are not distinguishable
from those of _A. sulcata_; but the adductor ridge is perhaps rather
more developed. The _Terga_ (Pl. 9, fig. 3 _c_) are nearly as large
as the scuta, and this is an unusual circumstance; the spur is more
than half as wide as the valve; it is placed not quite close to the
basi-scutal angle; on the carinal side, the basal margin of the valve
slopes a little towards the spur. I may mention, that in several
specimens from Madeira, the scuta and terga, on one side, had grown
to a monstrous thickness.
_Cirri_: these resemble, in every respect, those of _A. spongites_,
with the remarkable exception that on the anterior ramus of the
fourth cirrus, several segments were furnished with the beautiful
downward curved, mandible-like teeth, as in _A. sulcata_; but
differently from in that species, there were none on the upper
segment of the pedicel. I should have thought this an excellent
specific character, had not these teeth been so extremely variable in
_A. sulcata_.
Finally, I think this species is more nearly related to _A. sulcata_
than to _A. spongites_.