Tergum ([2 g]).—Spur of moderate length and breadth, with its lower end obliquely truncated and rounded. The longitudinal furrow has its edges somewhat folded in. The basal margin on the carinal side of the spur is sometimes a little hollowed out. The crests for the depressor muscles are well developed; but the corner of the valve supporting them is extremely thin, and is often imperfectly calcified.
Compartments.—The parietal tubes are not crossed by transverse septa, but in their upper parts are filled up solidly. The radii are always very narrow, with their summits oblique, though to a variable degree: their sutural edges have fine and closely approximate septa, with minute denticuli: the sutural edges are received in a furrow, on the opposed compartment, of unusual depth; hence the lines of suture run, in the lower part of the shell, almost exactly in the middle between each two compartments. The alæ are added to above the level of the opercular membrane.
The Basis is often thick, with an underlying layer, largely cancellated or honeycombed. When many specimens grow crowded together, the basis is generally deeply cup-formed, or even sub-cylindrical; and equals as much as four fifths of the length of the entire shell. In such cases, in some few recent specimens, and in all the large or even quarter-grown old tertiary specimens, but not in the quite young fossil specimens, a structure is presented, which I have not seen in any other Cirripede, namely, the basis (Pl. [4], fig. [2 a]) is filled up for one third, or even for more than half its depth, by successive, separate, calcareous, transverse layers or septa. It would appear as if the basal cup had grown too large for the animal’s body, and so required filling up. The layers are thin and fragile; a single layer never stretches across the whole shell; each is irregularly mammillated or blistered, with the convex surfaces generally directed upwards; the layers are furnished on their under sides with little pillars and short ridges, resting on the layers beneath; it rarely happens that the supports of one layer lie directly over those of another, though this is sometimes the case. In a vertical section, the mass formed by these irregular layers has a coarsely cancellated structure. This structure, although confined to this one Cirripede, is not so anomalous as might at first be thought, for in most species of the genus, each time that the circumference of the basis is added to, an excessively thin calcified film is thrown down over its whole inner surface; and in any of these species, if the films had been formed thicker and had rested only on certain points, instead of over the whole underlying layer, the cancellated structure above described would have been produced.
Mouth: the labrum is either destitute of teeth, or has two or three very minute teeth. The palpi have a tuft of very long spines at their ends. The third tooth of the mandibles is thicker and larger than the two upper ones. The maxillæ have either a nearly straight edge, or the inferior corner is obliquely truncated, and projects much beyond the rest of the edge. In the Cirri, none of the segments are very protuberant: in the first pair, one ramus is nearly twice as long as the other: in the posterior pairs, the segments are not much elongated, but each supports seven pairs of spines.
Var. nitidus: with respect to this variety I have little to add to my preliminary remarks on its peculiar appearance, owing to its smooth, naked condition, and pure white or pale purple colour. This colour, when examined through a lens, is seen to consist of very fine longitudinal stripes; and is produced by the calcareous matter within the longitudinal parietal pores being thus coloured. Generally the scuta have two longitudinal furrows; but I have seen a scutum of one perfectly characterised specimen with only a single broad furrow, like that which frequently occurs in the membrane-covered variety. Var. Coquimbensis, as before stated, differs only in its greater size: the scutum, in the one specimen examined, had two broad longitudinal furrows; neither it, nor the tergum differed from certain varieties now found on the coast of Chile.
17. [BALANUS] PERFORATUS. Pl. [5], fig. [1 a]-[1 d]; Pl. [4], fig. [3 a]-[3 c].
BALANUS PERFORATUS. Bruguière. Encyclop. Meth., 1789, Tab. 164, fig. 12 infra.
LEPAS ANGUSTA. Gmelin. Syst. Naturæ, 1789.
---- ORE ANGUSTIORE. Chemnitz. Vol. viii, Tab. 98, fig. 835.