Compartments, when the shell is very young, eight; when mature, either six, or in appearance only four owing to the close union of the lateral compartments: basis calcareous.
Distribution, Mediterranean, and New South Wales; deep water.
The two species here included form a very natural genus, though, as far as the shell alone is concerned, at first sight there is an unusual amount of difference between them. This genus offers an instance of a case, far from uncommon in nature, though so unfortunate for the systematist, in which the most obvious and useful characters of a group are completely masked. When I first examined [Pachylasma giganteum],[134] I did not doubt that it was a [Balanus]; and when I first looked at [P. aurantiacum], I thought, from there being in appearance only four compartments, that it was an [Elminius]; in neither case, from the absence of alæ to the rostrum, did I even suspect that the species belonged to the sub-family of the [Chthamalinæ]. But when I examined the included animal’s body, I found, in both species, the labrum bullate, not notched, with the palpi small, and the mandibles with their lower teeth not laterally double. Again I found in the third pair of cirri only the basal segments thickly clothed with spines; and lastly, there were caudal appendages. Now these characters are pre-eminently those of the [Chthamalinæ]; in fact, they are those met with in the typical genus [Octomeris], with the exception of the presence of caudal appendages, and these occur in [Catophragmus],—a genus standing next to [Octomeris], and in no other genus of sessile cirripedes. Moreover, if we look to the shell of [Pachylasma], the absence of pores in the parietes, or at least of symmetrical longitudinal ribs on their inner surfaces, and the peculiar character of the narrow radii, hardly differing in structure from the parietes, are characters which are rare in the [Balaninæ], but universal in the [Chthamalinæ]. Hence, taking the whole organisation of the two species of [Pachylasma], it is certain that they must be ranked amongst the [Chthamalinæ], though the leading character of the group, namely, the rostrum being furnished, like the carina, with alæ, here fails. Owing to this conviction, I examined very young individuals of [P. giganteum], and in specimens only the 1/100th of an inch in height, I was interested by finding eight separate compartments, with the rostrum having distinct alæ; hence, at this early age, as far as the shell is concerned, this species may be said to be an [Octomeris]; and we have seen that this likewise holds good with the included animal’s body; as the young shell increases in size the minute rostrum and rostro-lateral compartments blend together (Pl. [19], fig. [5 b]), without even traces of sutures being left.
[134] Dr. Philippi called this species a Chthamalus; giving this generic name from an examination only of the separated valves in a fossil condition.
With respect to [Pachylasma aurantiacum], I have no doubt that at an early age it would possess a perfectly distinct rostrum with alæ; for in the one specimen which I have seen, the compound rostrum is divided by sutures, faintly visible, both externally and internally (Pl. [20], fig. [1 a], [1 b]), into three compartments (B, A, B), of which the middle one, or true rostrum, still shows, in the manner in which it underlaps the little rostro-lateral compartments, vestiges of alæ. I may remark, that we have here the same structure as in [Chelonobia], formerly described, with the following differences, that here the sutures pass through the outer lamina of the parietes, so that, as seen externally, the separation of the three compartments is much more perfect than in [Chelonobia]; on the other hand, internally, the separation is less distinct, as the two rudimentary rostro-lateral compartments do not form part of the sheath. [Pachylasma aurantiacum] is further remarkable, from the two lateral compartments (C, D) on each side, tending to blend together, being only separated by sutures not more distinct, externally, than those separating the compound rostrum, but more distinct internally, for they run up the sheath. From these facts it follows, that this species, viewed outside, without particular care, would be said to consist, like an [Elminius] or [Chamæsipho], of only four compartments; if the sheath alone were looked at, there would be said to be six compartments; but when the walls, especially their basal edges, are carefully examined, either internally or externally, the eight compartments can be plainly distinguished.
With respect to the affinities of this genus, we have seen that both in the included animal’s body, and, at an early age, in the shell, it is extremely close to [Octomeris]: in the presence of caudal appendages, and in the basis being calcareous, we have a clear affinity with [Catophragmus]; in the two lateral compartments of [P. aurantiacum], tending to become blended together, we have some relationship exhibited to [Chamæsipho], as is likewise shown in the structure of the second and third pairs of cirri. If the genera of the [Chthamalinæ] were ranged in a circle, [Pachylasma] would be the point of contact with the [Balaninæ]. I must repeat, that it is extremely unfortunate that when the shell alone of [P. giganteum] is examined, it is hardly possible to separate this genus from [Balanus].
1. [PACHYLASMA] GIGANTEUM. Pl. [19], fig. [5 a]-[5 d].
CHTHAMALUS GIGANTEUS. Philippi (!). Enum. Mollusc. Siciliæ, 1836.
Shell and operculum dirty white: carino-lateral and lateral compartments furnished with similar alæ.