9. Genus—PLATYLEPAS. Pl. [17], fig. [1 a]-[2 b].
PLATYLEPAS. J. E. Gray. Annals of Philosophy, (new series), vol. x (1825).
CORONULA. De Blainville. Dict. des Sciences Nat. (1824).
COLUMELLINA. Bivona (fide Philippi). Nuovi generi, &c. di Molluschi, Palermo (1832).
Compartments six, each bilobed and inwardly produced, so as to form six midribs, which support the outwardly convex, membranous basis.
Distribution, throughout the tropical and warmer temperate seas. Imbedded in turtles, sea-snakes and manatee.
This small genus, consisting of three species (though I have named only two), is a very natural one, yet closely allied to [Coronula]. Most authors have united these genera, but in doing so they destroy two very natural little groups. [Platylepas], moreover, when all its characters are considered, has as good a claim to be generically separated from [Coronula], as has [Tubicinella],—a genus universally admitted. I shall presently recur to this subject.
General Appearance.—The most remarkable character, and which gives a peculiar aspect to the whole shell, is that each of the six compartments has a medial fissure or fold; the shell thus consisting of twelve lobes. Each fold extends from the summit to the base of the shell, and is produced inwards in the shape of a midrib (Pl. [17], fig. [1 a]). The membranous basis is supported by the basal edges of the six midribs, and is thus rendered convex. As the midrib on the rostrum would have interfered, if fully developed, with that part of the animal’s thorax which I have called the prosoma, it is of less size than the other midribs; and from this same cause the whole rostrum is pushed a little on one side, generally to the left, so that the shell is not perfectly symmetrical. The midrib of the carina is less than the lateral midribs. The orifice is oval, and is more or less plainly indented by six angular points,—the worn-down summits of the six midribs. The narrow elongated opercular valves stretch from one end of the orifice to the other. The rostrum is much broader than the carina: the lateral compartments are but little broader than the carino-lateral compartments. The radii are narrow, though variable in width. The general shape of the shell varies much in the same species, being either much depressed, or rather steeply conical; either circular, or more commonly oval. The (so called) epidermis is usually persistent in the lower part of the shell. The shell itself is white. The basal diameter of the largest specimen of [P. bissexlobata] was nearly three-quarters of an inch.
Structure of the Parietes, Radii, &c.—The walls of the shell in [P. bissexlobata] are permeated by minute pores, and have exactly the same structure as in [Coronula]; the pores being completed by the union of ledges on the outer sides of the longitudinal septa, and the latter have little knobs formed on them at each period of growth. In [P. decorata] the walls are solid. Each compartment, as already stated, has a deep nearly medial furrow, the sides of which are closely pressed together, thus forming the midribs, and thus differing from the furrows in [Coronula], which not being pressed together, form cavities on the under side of the shell. The inner longitudinal margin of each midrib is thickly coated by a layer of shell, and is thus rendered much more prominent than it would have been if formed exclusively by the inward folding of the wall. The upper part of each midrib forms a slight, longitudinal ridge (fig. [1 d]) on the sheath, having become encased by the sheath during its downward growth. Owing to the small size of the midrib of the rostrum, there is scarcely any ridge on the sheath of this compartment. The sheath does not descend half-way down the walls in [P. bissexlobata], but further down in [P. decorata]; but perhaps the thickened inner margins of the midribs may be considered as the downward prolongations of the sheath, and on this view the sheath descends actually to the basal membrane. The radii are narrow: their edges have simple septa, with the interspaces filled up solidly. The alæ have their sutural edges smooth.