The Basis is membranous, and more or less convex. This convexity is rendered possible by the support afforded by the basal edges of the six midribs. The so-called epidermis or membrane which covers the shell, is prolonged in six flaps under the midribs; and hence the proper basal membrane has six deep indentations, receiving the above flaps. The cementing apparatus is constructed on the same simple type as in [Coronula], as has been described in the [Introduction].
Scuta and Terga.—These are narrow and long, stretching the whole length of the orifice of the shell. The two valves touch each other, but are not properly articulated together: their upper surfaces scale off in layers: they resemble each other very closely in shape (fig. [1 c]), being simply oblong. The membrane covering the valves supports some very minute spines, 2/1000th of an inch in length. When a portion of valve is dissolved in acid it is seen to be penetrated by very numerous tubuli. The opercular membrane is rather thick: in [P. decorata] I found it occasionally double; in this species it supports some very minute spines; it is attached about one third down the sheath, of which the upper part is not marked (as in [Balanus]) by regular concentric rings. The aperture between the valves, leading into the sack, has protuberant lips, as in [Coronula] and [Tubicinella]. The normal five bundles of muscles surrounding the sack, differently from in the two genera just mentioned, do not thin out and spread out downwards, but retaining their full thickness, reach the basal membrane, to which they are attached.
Mouth: the parts present no difference from those in [Coronula]; between the outer maxillæ we have the same small flattened mentum-like projection; and in the mandibles, the same small intermediate teeth between the second and third, and between the third and fourth main teeth, as in that genus. The main teeth here are more plainly double laterally. There are teeth on each side of the central notch of the labrum. The cirri differ little from those of [Coronula], but the segments are not nearly so much flattened and broad, and the second and third pairs are not so short. The segments of the sixth pair support four pairs of main spines, with a few minute intermediate spines, but with no intermediate tuft.
Branchiæ: these consist on each side (at least in [P. decorata]) of a double fold, much less plicated than in [Coronula]; the fold nearest to the animal’s body is rounder, and not quite so large as the outer fold.
Range and Habits.—The [P. bissexlobata] ranges from the Mediterranean and west coast of Africa to the West Indies, to the north-east coast of Australia, and, judging from the name given by Chenu, to California: the second species, [P. decorata], inhabits the Pacific Ocean; and the third imperfectly known species, was deeply buried in the skin of a sea-snake, off Borneo. The [P. bissexlobata] is attached both to turtles and to the skin of the manatee: some specimens in the British Museum, attached to the latter, from the west coast of Africa, were entirely buried in the skin, with the exception of the operculum. The specimens of [P. decorata], which I obtained at the Galapagos Archipelago, were buried about half their depth in the softer parts of the skin of the green turtle. I do not doubt that the imbedment is effected in the same manner as in the case of [Chelonobia].
Affinities.—This genus, in general structure and habits, approaches closely to [Coronula]; but it differs from all the species of that genus in the following respects, which appears to me fully to justify its generic separation:—The outline is often oval, instead of circular; the compartments are not of equal sizes and shapes; each compartment has here only one inward furrow or fold, and this has its sides pressed closely together, and is encased, but not obliterated, by the sheath; the rostrum is not quite medial; and the radii are narrow and nearly solid. The opercular valves, again, are here more developed, and their microscopical structure is different. The opercular membrane is less constantly double; and, what is much more important, the muscles of the sack are not here spread out, and show no tendency to become rudimentary and lose their transverse striæ. The cirri differ only in not being so broad and flattened. The branchiæ are not quite so much developed. Finally, I may add, that in several of the above respects in which [Platylepas] differs from [Coronula], it approaches ordinary sessile cirripedes.