Var. (1), [3 c], [3 g]: tergum, with a sharp internal tooth, projecting rectangularly inwards.
Var. (2), [3 d]: tergum, with a broad blunt internal tooth, depending beneath the spur-like portion of the valve.
Var. (3), [3 f]: tergum, with the basi-carinal end of the valve truncated, with a small blunt internal tooth projecting rectangularly inwards.
Hab.—Red Sea; Mus. Brit. and Cuming. Also associated with [Pyrgoma crenatum], and attached to Meandrina spongiosa.[120]
[120] Mr. Dana informs me that he believes that this coral comes from the West Indies; though the specimens originally described by him had no label. If this be so, both [Pyrgoma dentatum] and [crenatum] have very wide ranges.
Appearance and Structure of Shell.—Shell nearly flat, oval, white or pink, with rather distant prominent ridges radiating from the moderately large (for the genus) oval orifice. The ridges are often obscured, and apparently sometimes almost obliterated by the encrusting coral. Shell permeated near the outer lamina by short imperfect pores: internal surface smooth: sheath inconspicuous, descending rather more than half way down the walls; lower edge closely attached to the walls. Basis deep. Diameter of largest specimen .3 of an inch.
Scuta: these are elongated, but to a very variable degree, some specimens being quite three times, and some barely twice as broad as high. I observed this same variability in two sets of specimens, differing, as we shall presently see, in the form of their terga: it depends in part, but not wholly, on the varying width of the occludent ledge, which is sometimes only a fourth, and sometimes half as high as the rest of the valve. This is the first species in the genus in which the occludent ledge—a structure peculiar to the genus—has been amply developed. The basal margin of the valve is slightly sinuous, and a very little hollowed out near the basi-tergal corner; it is also very slightly reflexed, the reflexed portion being separated from the upper part of the valve by a very slight depression or even furrow. I notice this slightly reflexed portion, simply as indicating a well marked feature in the basal margin of the following closely allied species. Internally ([3 g]), the adductor ridge is thick and slightly prominent, but does not descend beneath the basal margin: it blends into the articular ridge, which here projects in a remarkable manner and degree ([3 b], [3 e], [3 g]), like a rounded tooth. This tooth is in part a development of the occludent ledge; it varies much in shape. The line of junction between the scutum and tergum is nearly straight, and nearly at right angles to their longer axes. In some specimens the scuta and terga are partially calcified together.
Terga: in three sets of specimens the terga differed considerably, but as in every other respect there was the closest resemblance, I do not doubt that these are merely varieties. In all three, the valve is rather small, irregularly sub-triangular in shape, and externally somewhat convex; in all three, there is an occludent ledge, of variable width as in the scuta; and in all three, there is an internal tooth-like projection, of variable form, unlike anything I have seen in any other cirripede. In the first variety (Pl. [13], fig. [3 c], [3 g]), the basi-carinal corner of the valve is bluntly pointed, and a slight flexure separates this portion of the valve from the other and scutal half, which latter thus exhibits some tendency to be converted into a spur: on the internal surface ([3 g]) of this spur-like portion of the valve, there is a rather long, sharp tooth, which projects rectangularly inwards; it is flattened in a plane at right angles to the longer axis of the scutum and tergum together: it cannot be seen from the outside. In the second variety, the shape of the valve is not very different (Pl. [13], fig. [3d]), excepting that the flexure, separating the basi-carinal corner of the valve from the spur, is deeper; but on the internal face of the spur, the tooth is far broader than in the first variety, and is flattened quite differently, viz., in a plane nearly parallel to the surface of the valve, and instead of projecting rectangularly inwards, it depends beneath the basal edge of the so-called spur, and can be seen from the outside. In the third variety (fig. [3 f]), the whole carinal end of the valve is cut off, and there can hardly be said to be any trace of a spur, yet a slight furrow apparently marks the line of separation between the basi-carinal portion of the valve, here become very narrow, and the broad, irregular part, which would have formed the spur had such been developed: on the internal surface of the latter portion of the valve, there is a very small, blunt, slightly flattened tooth, projecting inwards, and more resembling that in the first than that in the second variety.
Affinities: under [P. crenatum] I will point out the diagnosis and relationship between this and that species.