The Basis is entirely membranous, in which respect this species differs from all the others in the genus.
Mouth: all the trophi are unusually hairy or spinose. The labrum is deeply notched and apparently destitute of teeth on the crest. The palpi are club-shaped or enlarged at their extremities. The mandibles have the fourth tooth rudimentary.
In the Cirri, the second and third pairs are remarkably short and blunt. In one specimen the two rami of the first cirrus had respectively six and sixteen segments; those of the second, six and seven; those of the third, seven and seven; and those of the sixth cirrus twenty rather elongated segments, with a small tuft of spines between each main pair of spines.
Affinities.—This species differs from all, in its membranous basis, and in its transversely elongated scuta. It resembles [T. costata], and differs from all the other species, in its radii (when developed) being square on the summit, and in being formed of tubes,—in the smallness and number of the parietal tubes,—in the absence of crests on the under side of the scuta for the rostral and lateral depressor muscles,—in the shortness and rounded form of the spur to the terga,—and, lastly, in the club-shaped palpi and small size or absence of the fourth tooth in the mandibles. [T. purpurascens] differs from [T. costata] in those points, namely, in its membranous basis and transversely elongated scuta, in which it differs from all the other species, and, moreover, in its scuta not being plainly striated longitudinally, in having more ribs on the external surface of the parietes of its shell, and in having only three pairs of main spines on the three posterior cirri.
5. [TETRACLITA] COSTATA. Pl. [11], fig. [2 a]-[2 c].
Shell depressed, whitish, generally with ten very prominent longitudinal ribs: radii broad, with their summits parallel to the basis: basis calcareous: scutum externally striated longitudinally: tergum with the spur short and rounded.
Hab.—Philippine Archipelago, Mus. Cuming. Attached to various shells, within the tidal limit.
General Appearance.—Shell whitish, probably tinged, when alive, with reddish-purple; depressed; surface perfectly preserved, smooth, but having longitudinal very prominent ribs, almost invariably ten in number; namely, three on both the rostrum and carina, and two on the two lateral compartments, with ten corresponding projections round the basal margin. Orifice passing from rounded-trigonal to diamond-shaped. The radii are very broad and square at the summit, and extend from tip to tip of the compartments. Basal diameter of largest specimen under half an inch, generally from .3 to .4 of an inch.