General Appearance.—Shell rather elongated or tubular; with the upper part reddish, and the surface roughened with very minute points. The basal cup is generally as deep as the shell is high, ending downwards in a blunt point, often curved to one side. The summits of the radii, as usual, are oblique. The parietes of the carino-lateral compartments are about half as wide as the parietes of the lateral compartments, and are therefore of greater proportional width than in the foregoing or any other species of the genus. The large membrane-covered openings, or, as they may be called, windows, presently to be described, between the lower halves of the compartments, is much the most remarkable character of this species. The largest specimen which I have seen was only .23 in diameter, and .6 of an inch in height, measured from the basal point of the cup to the tips of the compartments.
Scuta.—These barely exhibit a trace of longitudinal striæ. The valve is rather thick and convex. The basi-tergal angle is much rounded off. Internally, the articular ridge is thick and rather prominent. Terga: the valve is furrowed in the line of the spur: the spur is pointed and rather long; it is distinctly separated from the basi-scutal angle of the valve, and the basal margin on the two sides of the spur forms a straight line. The articular ridge is prominent, and short.
Structure of the Parietes, Radii, and Basis.—The parietes are internally quite smooth down to its basis. The edges of the radii are also smooth, as is the edge of the basal cup. The alæ project less than usual. The radii are of moderate breadth, they extend downwards only a little below the sheath, namely, about half way down the shell, where they terminate, as usual, in a point. The increase in width, during growth, of the radii, and their not extending down to the basis, would necessarily cause a gap between the opposed edges of the walls, in the portion beneath the radii; but besides this, the edges of the walls themselves, beneath the radii, and on the opposed side beneath the alæ, are hollowed out, but on the latter side or beneath the alæ sometimes in a lesser degree. The result of this is, that the compartments, in their lower halves, are separated from each other by membrane-covered windows or apertures, arched at their upper ends, and of considerable size, namely, about as wide as the parietes of the carino-lateral compartments. I have only further to remark, that during the downward growth of the parietes, the apertures increase in size, but at the same time become closed up at their upper ends; and the arched layers of shell added at these upper ends, assume a very different aspect from the rest of the parietal surface,—appearing like two wedges, with their points upwards, let in, on one side of the suture, between the ordinary parietal surface and the radius, and, on the other side of the suture, between the ordinary parietal surface and the recipient furrow of the radius.
The animal’s body was in a bad state of preservation; but, as far as I could make out, the cirri resembled those of [A. glans].
Affinities.—This species differs from [A. glans] in not having the internal margins of the compartments projecting inwards. It differs from all the ordinary varieties of [A. spongites], in the smoothness of the basal edges of the parietes and of the edge of the cup; in the greater width of the carino-lateral compartment, though this is a variable point in [A. spongites]; slightly in the shape of the scuta and terga; and, lastly, in the large, membrane-covered openings between the compartments.
8. [ACASTA] PURPURATA. Pl. [9], fig. [8 a]-[8 c].
Shell dull blueish-purple, with six small, membrane-covered apertures between the sutures, close above the basis: tergum with the articular ridge very short and prominent; spur very broad and rounded.
Hab.—Sumatra; Philippine Archipelago; imbedded in the bark of an Isis; Mus. Cuming, Stutchbury, Brit.