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.
3. Genus—TETRACLITA. Pl. [10], [11].
TETRACLITA. Schumacher. Essai d’un Nouveau Syst., &c., 1817.
CONIA. Leach. Journal de Physique, tom. 85, 1817.[105]
ASEMUS. Ranzani. Memoire di Storia Naturale, 1820.[106]