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.
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.
CORONULA BISSEXLOBATA. De Blainville. Dict. des Sciences Naturelles, tom. 32, (1824), Tab. 117, fig. 1.
PLATYLEPAS PULCHRA. J. E. Gray (!). Annals of Philosophy, (new series), vol. 10, (1825).
COLUMELLINA BISSEXLOBATA. Bivona (fide Philippi). Nuovi generi di Mollusch. (1832), Tab. 3, fig. 1.[126]
CORONULA CALIFORNIENSIS. Chenu (!). Illust. Conch., Tab. 1, fig. 4.
[126] This memoir was published in the ‘Effemeridi Scientifiche e Litt. per la Sicilia,’ according to the ‘Bibliographia Zoologiæ et Geologiæ,’ by Agassiz and Strickland.
Shell with the transverse lines of growth conspicuous: parietes permeated by pores; sheath descending barely half-way down the parietes.
Hab.—Mediterranean, attached to turtles. River Gambia, attached to manatee. Honduras, attached to manatee. Moreton Bay, lat. 27° S., Australia, apparently attached to the dugong of that coast. California(?) Mus. Brit., Stutchbury, and Cuming.
General Appearance.—Shell generally much depressed, and broadly oval or circular; sometimes steeply conical. Orifice oval, generally not large. Surface rather plainly marked by closely approximate lines of growth, which with the lobed outline gives to the whole an elegantly sculptured appearance: occasionally the longitudinal ridges formed by the parietal septa are distinct. Basal diameter of largest specimen three quarters of an inch.
Structure of Shell.—I have nothing material to add to the generic description. The midribs are not so prominent as in the following species, and hence the basal membrane is less convex. The origin of the midrib as a fold is very plain. The sheath descends barely half-way down the walls, and is a little hollow on its under margin, on each side of the midrib. Beneath the sheath the parietes are finely ribbed (fig. 1 d), but to a variable degree. I may here remark, that in the specimens taken from manatee, on the coast of Africa and at Honduras, the internal ribs extended further up and were plainer, and the opercular valves seemed to be a little narrower than in the other specimens, so that I at first suspected that they were specifically distinct, but I could make out no other than these small and variable points of difference.
Scuta: oblong (fig. 1 c), about twice as long as broad, with the rostral end rounded, rather narrower than the other end, and curled a little inwards. Terga, of nearly the same shape and nearly as long as the scuta: the carinal end is rather more pointed than the scutal end of the valve, and when viewed internally, the growing surface of this end is seen to be bluntly pointed. In both valves the upper layers of shell usually scale off.
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.
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.
Hab.—Imbedded in the skin of a sea-snake, taken off Borneo.
I am indebted to Dr. Gray for a single specimen of this supposed species, but as it is very young and imperfect, wanting the opercular valves and cirri, I do not choose to name it. The shell presents all the usual characters of the genus; the rostrum, I may remark, being pushed to the left side. The parietes are permeated by pores of considerable size, which shows that the species is distinct from P. decorata. On the inner basal surfaces of the walls, there are two or three very distinct ridges on each side of the midrib; and this fact, together with the size of the parietal pores, makes me suspect that it is not an immature specimen of P. bissexlobata.