Terga, with the spur placed close to the basi-scutal angle, so that there is no basal margin on that side; spur short, with its lower end truncated and rounded; broad, even exceeding, when measured across the upper part, half the width of the valve. Articular furrow wide. Apex not beaked.
Structure of Walls and Radii.—This species differs from all the others of the genus in having only a single row (fig. [3 d]) of parietal tubes; these are large, quadrangular, but elongated in the ray of the circle. They are not filled up, even at the very top of the shell, but they become thickly lined all round with compact shelly matter. When the surface of the shell is disintegrated, these upfilled tubes greatly affect, as already stated, the external appearance. The outer lamina near the basis is internally strengthened by longitudinal, sharp, approximate ridges or plates, which, also, often affect, after corrosion, the external appearance. The radii have their sutural edges formed by a set of narrow, branching ridges or septa; the ends of which, seen externally, often give a notched outline to this edge; the recipient furrows in the opposed compartments are deep, and their edges likewise are often notched: the interspaces between the branching ridges are filled up solidly. The alæ have their edges coarsely crenated. The lower edge of the sheath is not free.
The mouth and cirri present no particular characters: the third cirrus has both its rami elongated, with the terminal segment tapering. In the three posterior pairs of cirri, the tufts of little spines between the main pairs are rather large.
Affinities.—This species has no particular affinity with any other. The circumstance of there being only a single row of parietal tubes is not so important a difference as might at first be thought, inasmuch as in the other species, during their quite early youth, the walls are formed of only a single row of tubes or pores.
4. [TETRACLITA] PURPURASCENS. Pl. [11], fig. [1 a]-[1 d].
LEPAS PURPURASCENS.[110] Wood’s General Conchology, p. 55, Pl. 9, fig. 42, 1815.
BALANUS PLICATUS. Lamarck. Animaux sans Vertèbres, 1818.
------ ------ ET PUNCTURATUS. Chenu. Illust. Conch., Tab. 4, fig. 3 et 12.