Hab.—Philippine Archipelago; Mus. Cuming.
General Appearance and Structure.—Shell depressed, with a large diamond-shaped orifice. Colour beautiful violet-purple, but externally much obscured by disintegration, causing the shell to be ashy gray. Walls smooth or slightly folded. Sutures generally quite, or almost quite obliterated; but when well preserved, they differ remarkably in appearance from those in the foregoing species; for the radii externally here consist of oblique plates or laminæ arising on both sides of the sutures, standing nearly parallel to the parietes, and interfolding with each other. These laminæ are rather plainly marked by lines of growth. Essentially the radii do not differ much from those in [C. dentatus] and [Hembeli]; we have but to produce obliquely upwards the transverse and interlocking ribs on their radii, and so convert them into laminæ. During the diametric growth of the shell, the sutural edges of the alæ are added to, in the usual manner, by upturned lines of growth; and, in addition, the recipient furrows of the alæ are similarly added to, so that the lines of growth are upturned, and alæ appear to have been developed on both sides of the sutures in the same way as the radii appear to have been developed on both sides in many [Chthamalinæ], though rarely in the [Balaninæ]. The inside of the shell is beautifully coloured rich violet; it is punctured with small holes as so often is the case with [C. stellatus]. In every specimen (all full-grown) which I opened, the inner basal edges of the parietes were inflected rectangularly inwards, forming a smooth-edged ledge all round the basal membrane, which, in proportion to the width of this ledge, was by so much reduced in diameter. The largest specimen which I have seen was .35 of an inch in basal diameter.
Opercular valves.—The scuta and terga, in all the specimens which I have seen, were firmly calcified together; in some, a trace of a suture could be seen externally, but hardly a trace internally. In one specimen, there were vestiges of some impressed lines on the scutum, in exactly the same position in which such occur in [C. Hembeli]. The scutum is rather narrow. The basal margin of the tergum is either straight, or depends a little on the scutal side, thus producing a small spur: the crests for the depressor muscles are strongly marked, and depend beneath the basal margin.
Mouth.—The labrum is strongly toothed: the palpi have long hairs along the exterior basal margin: the mandibles have only three main teeth, and the inferior coarsely pectinated portion is short; the maxillæ deeply notched. Cirri: the first and second pairs have their rami slightly unequal in length; the third pair differs from the same pair in the other species of the genus, in having some few of the basal segments on the anterior ramus thickly clothed with spines, so as to be brush-like: there is even a trace of a similar structure in the lowest segments of the posterior ramus. In the three posterior pairs of cirri the segments are much elongated, and support four pairs of spines.
Branchiæ.—Unknown.
8. [CHTHAMALUS] SCABROSUS. Pl. [19], fig. [2 a]-[2 d].
Shell (when well preserved) dull purplish-brown: sutures formed by oblique interfolding though rarely well developed: tergum with a deep narrow pit, at the basi-carinal angle, for the depressor muscle.
Hab.—Peru, Chile, Tierra del Fuego, Falkland Islands. Very common; attached to littoral rocks and shells, and often associated with [Balanus flosculus], and sometimes with [Chthamalus cirratus]; Mus. Brit., Cuming, W. Dunker, Darwin.