12. Genus—CHTHAMALUS. Pl. [18], [19].
CHTHAMALUS. Ranzani. Memoire di Storia Naturale, 1820.
EURAPHIA. Conrad. Journal Acad. Nat. Sc. Philadelphia, vol. 7, 1834.
Compartments six: basis membranous, but sometimes in appearance calcareous, owing to the inflected parietes.
Distribution mundane; attached generally to littoral rocks and shells.
This, the typical genus, is the largest and widest distributed group of the sub-family [Chthamalinæ]. The founder of the genus apparently did not perceive its essential character; Savigny, however, as is evident from the excellent figure in the great work on Ægypt, perfectly understood the difference between [Chthamalus] and [Balanus]. I was first indebted to Dr. J. E. Gray for explaining to me this difference; but the only published account which I have met with is in a paper by the Rev. R. T. Lowe,[130] in which he states, on the authority of Mr. Clark of Bath, that in [Chthamalus] the anterior compartment or rostrum has alæ like the posterior compartment or carina, the anterior or rostro-lateral compartments being destitute of alæ. These characters being exactly reversed in [Balanus], as I have already explained (p. [176]) under the sub-family of [Balaninæ].
[130] ‘Zoological Journal,’ vol. 3, p. 76, 1828.
The shell, owing apparently to its containing much animal matter, is particularly subject to disintegration; and when thus much affected it is quite impossible to distinguish the species by external characters. It is, in fact, best to cast on one side external appearance, though when the shells happen to be well preserved, each species has its own peculiar aspect. We have in this genus smooth and plicated, cylindrical and depressed varieties of most of the species. The development of the radii is very apt to vary, and even the compartments often become so completely united and calcified together that the sutures are almost or quite obliterated. A more serious difficulty in discriminating the species, arises from the fact of the opercular valves, not only varying extremely in external appearance in consequence of the greater or less disintegration of their apices, and consequent exposure of their articular ridges and furrows (compare fig. [1 a], [1 b], [1 c], in Pl. [18]), but from their truly varying in outline with the varying shape of the shell: this latter circumstance is probably due to the opercular membrane which unites the valves to the shell being very narrow, and in consequence, differences in the shape of the shell affect the opercular valves, in a manner and to a degree to which the [Balaninæ] are not subject (compare fig. [1 e], [1 f], [1 h], in Pl. [18]). The scuta, on the other hand, differ to an unusually slight degree in the different species. In the common [Chthamalus stellatus], which abounds on the southern British shores, the whole external aspect of the shell is often so completely masked, owing to its varying shape, its obliterated sutures, its deeply disintegrated and punctured surface, and by the corroded condition of its opercular valves, that I have found this species, in the collections of naturalists who have attended to cirripedes, arranged actually on the same tablet, mingled with specimens of [Balanus balanoides]. I have myself several times found it less troublesome to discriminate these two genera by the included animal’s body than by the shell,[131] though the latter, when well preserved and developed, possesses such obviously well-marked characters: the same thing has occurred to me with some of the other species.
[131] It will be found ultimately to save time and trouble, to soak for half an hour in hot caustic potash, a specimen out of each group of shells to be examined, and then well wash and brush the separated valves and compartments; this process has been followed by me with all the species here to be described.
General Appearance.—The shell is generally depressed, but sometimes, when growing in groups, cylindrical. The surface is either smooth or longitudinally folded; and, as already stated, very apt to be deeply disintegrated. The radii, when developed, are narrow, with their summits oblique and rounded; but they are often quite absent, and sometimes even the sutures are almost obliterated. The rostrum and carina are of the same shape and size, and the two lateral compartments on each side are of nearly equal breadths. The orifice is generally sub-rhomboidal, being widest towards the carinal, instead of towards the rostral end, as is usual in [Balanus]: but in [Chthamalus fissus] the orifice is narrow and elongated. The opercular valves have their apices generally disintegrated and worn away, and are then seen to be deeply locked together. The colour of the shell is dirty white or dull purplish-red or brown; but in [C. intertextus] rich violet-purple. The species are small, not often exceeding half an inch in basal diameter, with the exception of [C. Hembeli], of which I have seen a specimen two and three quarters of an inch in diameter.