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].
4. [ACASTA] UNDULATA. Pl. [9], fig. [4].
Shell, apparently, as in [A. spongites], but larger: scutum marked by longitudinal ridges, often in pairs, with the intermediate furrows rather wide: spur of tergum nearly half as wide as valve.