14. [BALANUS] TRIGONUS. Pl. [3], fig. [7 a]-[7 f].
Parietes ribbed, mottled purplish-red; orifice broad, trigonal, hardly toothed. Scutum thick, with from one to six longitudinal rows of little pits. Tergum without a longitudinal furrow; spur truncated, fully one third of width of valve.
Hab.—Java; East-Indian Archipelago; Peru; West Columbia; California; Sydney; New Zealand. Mus. Brit., Cuming, Stutchbury, Dunker, &c.
General Appearance.—Shell conical, generally depressed; orifice broad, triangular, almost equilateral; walls coloured or only mottled with purplish-pink, having either irregularly branching, or regular, longitudinal ribs, which are generally white. The radii are pale pink, or nearly white: the opercular valves have either their upper parts, or nearly their whole surface, clouded with pinkish-purple: the epidermis is not persistent: the walls are moderately strong: the largest specimen was one inch, but generally full-grown specimens are about half an inch in basal diameter.
The Scuta have the lines of growth highly prominent. From one to five or six rows ([7 b], [7 c]) of nearly circular, or transversely oblong, deep pits, extend down the middle of the valve; rarely there is not even one row; in this latter case, the valve is not striated longitudinally. These little pits are caused by one or more deep longitudinal furrows, crossed by the lines, or rather ridges, of growth. In the same group of specimens, I have seen individuals with three, five, and six rows; and even a few specimens with only one row, or none at all. The outline of the valve is elongated, with the apex slightly reflexed: the inner surface is protuberant, sometimes to a remarkable, but variable degree. The articular ridge is not very prominent, but it extends fully half-way down the valve, and generally ends in a small free point. There is a short adductor ridge, and a deep narrow pit or cleft for the lateral-depressor muscle. Terga, externally smooth, flat, with scarcely a trace of a longitudinal furrow; spur broad ([7 e], [7 f]), varying from half to one third of the width of the valve, with the end truncated, situated either near or quite close to the basi-scutal angle. The crests for the depressor muscles are moderately well developed.
Compartments.—The parietal tubes are, in their upper parts, filled up solidly, without transverse septa. The radii generally have their summits slightly oblique, and this is almost always the case with the radii of the rostrum; the other radii sometimes extend from tip to tip of the parietes, and are parallel to the basis; rarely the radii are considerably oblique. The septa of the radii are very obscurely denticulated, and the interspaces between them are filled up solidly. The alæ have their sutural edges thin and smooth.
Mouth.—Labrum with three teeth close together on each side of the central notch: mandibles with four teeth, the fourth being small, the fifth either absent or scarcely distinguishable from the inferior angle: maxillæ without any notch, with the two lower spines rather longer than the others. Cirri: In the first pair, one ramus is only half the length of the other; in the second pair, both rami are short and about equal in length; in the posterior pairs, the segments, which are not protuberant, bear four pairs of spines, of which the three lower pairs are short.
This species is widely-distributed, and where found seems to be common. It is generally attached to shells of mollusca, but I have seen it also attached to wood. I have found it associated with [B. tintinnabulum], var. concinnus, and coccopoma, with [B. psittacus], [improvisus] and [amphitrite], and with [Elminius modestus].
Young specimens bear a considerable resemblance to certain young varieties of [B. tintinnabulum], and can indeed be distinguished from them only by a careful examination of the opercular valves; for it should be borne in mind, that in certain cases the scuta in [B. tintinnabulum] are pitted with little cavities. This species in some respects is, I think, allied to [B. porcatus], but it is far more closely related to [B. spongicola], and can be discriminated with difficulty from certain varieties of this latter species. In Mr. Cuming’s collection, there is a group of small specimens, crowded between some older specimens, which are remarkable from the shell being oval in a transverse section,—from the smoothness of the walls,—and from the absence of pits on the scuta; yet there could be no doubt that these specimens belonged to our present species.