Shell with the upper part tinged greenish-blue, longitudinally ribbed: radii moderately wide, with their summits oblique: scutum with a small adductor and extremely prominent articular ridge, united together and so forming a small sub-cylindrical cavity: tergum with the spur not joined to the basi-scutal angle.
Hab.—Philippine Archipelago, attached to [Balanus tintinnabulum]; attached to a ship’s bottom, and to [Balanus tintinnabulum], both from the Pacific Ocean; attached to a massive coral, and associated with [T. vitiata], and therefore from the tropical eastern seas; Mus. Brit., Cuming, Stutchbury.
General Appearance.—Shell conical, sometimes depressed; surface with rather broad, smooth, longitudinal ridges; whitish, with the upper part greenish-blue, sometimes very feebly tinted with pink; radii white, or mottled with blueish-green, or with pink. When the outer lamina of shell has been corroded, the upfilled parietal tubes, of a dull blueish-gray colour, are exposed. The radii are moderately wide, with their summits very oblique. In basal diameter, one specimen was 1.8, and another 1.5 of an inch.
Scuta, externally furrowed very slightly in a longitudinal direction, causing the lines of growth to become a little sinuous. The valve is strong and thick; and the epidermis, when preserved, is hirsute with spines. The articular ridge is extraordinarily prominent; it projects, as measured from the external surface of the valve, to an amount equalling half the width of the valve in its widest part. The adductor ridge is very short, and is united to the bottom of the articular ridge, thus forming a small, nearly cylindrical tube, which runs up to near the apex of the valve. The inflected occludent margin is broad and coarsely toothed. The crests for both depressor muscles are not very prominent. When the scuta and terga are articulated together, owing to the great projection of the articular ridge of the scutum, its upper part is separated from the tergum (fig. [4 b]), by a remarkably wide and deep, fissure-like hollow. The scutum, in some distorted specimens from a ship’s bottom, was narrow in proportion to its ordinary breadth.
Terga.—These are large. Externally, there is a broad, longitudinal depression, bounded on each side by a ridge. The carinal half is feebly striated longitudinally. A large upper portion (fig. [4 b]) projects freely, and stands, when the two valves are articulated together, above the apex of the scutum, but does not form a beak as in [T. porosa]. Internally, the tergal margin is widely inflected; the articular furrow is very deep, but the articular ridge is not prominent. The spur is not very broad; it is separated from the basi-scutal angle by a small space of basal margin, which forms a straight line with the basal margin on the opposite side of the spur. The end of the spur is truncated, and is parallel to the basal margin.
Structure of the Shell and Radii.—The walls are thick: the parietal tubes are rather large and regular; they become solidly filled up in their upper parts. The sutural edges of the radii are formed by unusually narrow, sinuous ridges, sending off delicate denticuli on each side: the interspaces between these ridges are solidly filled up. The crenated edges of the alæ are rather thick. Neither the mouth nor the cirri offer any noticeable character; but I may observe, that, in the mandibles, the third and fourth teeth are close together; and that, in the three posterior cirri, the tufts of small spines between the main pairs are small.
Affinities.—This species is very distinct from the others, with the exception of the following [T. radiata], to which it is in several respects allied. The under side of the scutum, with its great articular ridge, and the cylindrical tube formed by the union of the latter with the short adductor ridge, affords the most noticeable character.
8. [TETRACLITA] RADIATA. Pl. [11], fig. [5 a]-[5 d].
CONIA RADIATA.[112] De Blainville. Dict. Sc. Nat., 1816-1830, Pl. 164, fig. 5, 5 a, (sine descript.)