Mouth and Cirri. The only differences, compared with the following species, that I perceived, were that the hairs at the end of the palpi were here rather longer, and the fourth tooth in the mandibles rather less distinct. The rami of the first cirrus are very unequal in length.
2. [PLATYLEPAS] DECORATA. Pl. [17], fig. [2 a]-[2 b].
Shell with fine longitudinal ridges, ornamented in the lower part by minute beads; parietes not porose: basal membrane equalling in convexity the shell.
Hab.—Pacific Ocean; Galapagos Archipelago; Lord Hood’s Island, Low Archipelago; Mus. Cuming, Darwin.
General Appearance and Structure.—Shell oval, with the orifice large. The walls are thick, and of less height from top to base than in the last species. The surface (and this is the chief external character) is marked by fine longitudinal ridges, each of which, when closely examined, is found to be double (fig. [2 b]), and near the basis, where not abraded, to be ornamented with minute beads on each side: these beads are largest on those ridges which border the sutures. The parietes are not permeated by pores; on their inner surfaces there are a few rather prominent but short ridges, instead of the many finer ridges, as in [P. bissexlobata]. The sheath is of great thickness; in young specimens layers scale off its summit, all round the shell, as in [Tubicinella]; it is only slightly hollowed out at its lower margin; it descends more than half-way down the walls. The midribs, formed by the folded walls, are more prominent than in the last species, and descend lower down; hence the basal membrane is more convex than in the last species, for it projects downwards as much as the shell projects upwards. In medium-sized specimens, the midribs project inwards to a distance exceeding half the breadth of a compartment. A less proportional length of the midrib, in an inward direction, is formed by the inward folding of the wall, and a greater length by the thickening of its inner longitudinal margin, than in the last species. The basal edge of the inner end of the midrib is smooth.
Dimensions.—The average size of the numerous specimens which I obtained at the Galapagos Islands was about .2 of an inch in diameter, and I found none larger; but Mr. Cuming’s specimens from Lord Hood’s Island are half an inch in diameter.
Opercular Valves.—These closely resemble those of the former species, with the exception that the carinal end of the growing or under surface of the Tergum is much squarer.
Cirri.—The only difference which I could perceive was, that the rami of the first pair were not quite so unequal in length.