Shell with the orifice narrowly ovate: sheath dark purple: Scutum much elongated: tergum triangular, convex, without a spur.
Hab.—Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus. Brit., Stutchbury, &c. Imbedded in Millepora complanata, sometimes associated with [Balanus Ajax].
Appearance and Structure of Shell.—Shell oval, flat, coloured pale dull purple, or white, with slight and narrow ridges, radiating, from the orifice, which is not quite central, but is placed rather nearer to the carinal than to the rostral end of the shell. The orifice is small and narrow; the carinal end being rounded, and the rostral end narrower and sharper,—this being the exact reverse of the usual shape of the orifice in the [Balanidæ]. The walls are thick, and are formed of large square tubes. The internal surface of the shell is smooth. The sheath (fig. [2 b]) is much more elongated than the shell, for at the rostral end it extends to the basal margin, and at the carinal end nearly to it, whereas on the two sides it is some way distant from the base. The orifice is considerably out of the centre of the sheath, being placed nearest to the carinal end. The sheath has its lower edge slightly prominent or free; the lines of growth are neither turned up nor down on the two sides, as in the last three species. When nearly full grown the sheath is coloured dark purple, but when young it is white, hence the upper part is white, surrounded by an oblong purple ring, and this is surrounded by the white shell. The basal cup is deep, and internally nearly smooth. The largest specimens were .3 of an inch in their longer diameter. Great masses of the Millepora are absolutely studded with this [Pyrgoma], with usually more specimens on one side of the plate or branch than on the other. They stand in approximately parallel positions, the broad or carinal ends of the orifices pointing upwards.
Scuta.—The scuta and terga are closely united, and are often (perhaps always) in some slight, though variable degree, calcified together; and hence they often break, rather than separate, at the line of articulation. The external fissure or line of junction between them (fig. [2 c]) is oblique to the longer axis of the scutum; in the uppermost part of the valve it is sometimes almost obliterated. The two valves together are nearly as long as the sheath, and consequently much longer than the orifice of the shell. The scutum is much elongated, being fully four times as broad as high. The valve narrows towards the rostral end, but in a variable degree: the basal margin is hollowed out a little (but to a variable amount), close to the basi-tergal corner. Along the occludent margin a slip of the valve, widening downwards, is a little bent inwards, and this inflected portion is separated from the rest of the surface by a slightly angular ridge, running from the apex to near the rostral angle. Internally, at the basi-tergal corner, a slight ridge, parallel and close to the basal margin (and which can be seen only when the basal edge of the valve is held upwards), represents the adductor ridge, which we have seen so largely developed in the last three species, and shall again see in [P. crenatum]. The articular ridge (fig. [2 d]) is extremely prominent, consisting of a more or less rectangular shoulder.
Terga: these are rather small compared to the scuta: they are triangular and much arched: there is no trace of a spur. Internally (fig. [2 f]), the articular ridge is central: there are some vestiges of crests for the depressor muscle.
Affinities.—This well-marked species, in the tendency of the opercular valves to be soldered together, and in the remarkable absence of a trace of a spur to the tergum, is allied to [P. grande], but it is more closely allied to the three following species.
7. [PYRGOMA] DENTATUM. Pl. [13], fig. [3 a]-[3 g].
Scutum much elongated, with a tooth-like articular projection: tergum convex, irregularly triangular, sometimes with an imperfect spur, and on the internal surface with an inwardly projecting tooth; scutum and tergum furnished with an occludent ledge.