Variation.—The species vary in shape nearly, but not quite as much as in [Balanus], for we very rarely here see cylindrical varieties; but in the same species, we have extremely depressed, steeply conical, and convex forms; the orifice varies in relative size; the state of the surface, whether ribbed or smooth, whether well preserved or corroded, the upfilled parietal tubes being thus exposed, varies more than in [Balanus]. The colour also varies; specimens of [T. porosa] (Pl. [10], fig. [1 a] to [1 f]) being dark purple, or even inky black, white, pale pinkish-purple, and green: as far as I have seen, the pinkish varieties of [T. porosa] are confined to the eastern half of the world. Even the ridges and crests on the under side of the scutum vary in some degree; and the tergum in [T. porosa] varies considerably, and in some of the other species, slightly, in general shape: I believe that the tergum becomes narrow and elongated, when the shell is steeply conical, with the orifice of small diameter. After having spent several days in disarticulating and closely examining the many specimens of [T. porosa] in my possession, I concluded that its varieties formed at least four species, these being in external appearance extremely distinct; but ultimately the many intermediate forms compelled me to unite all into a single species. Again, I may instance the conical, ribbed variety of [T. purpurascens] (Pl. [11], fig. [1 b]), with the outer lamina of the shell preserved and with the radii widely developed, as having not the smallest resemblance to the other common depressed variety (fig. [1 a]), having a granulated surface, produced by the exposed tips of the upfilled parietal tubes, and without a trace of the radii or even of the sutures; I may add that, according to the characters used by some authors, these two varieties would be classed in two separate genera.
To distinguish the species of this genus, the chief reliance must be placed (as in the case of most other sessile cirripedes) on the general outline of the opercular valves, and on the ridges and crests on their under sides: the internal structure, however, of the radii, and in a lesser degree of the parietes, must not be overlooked. I have not found the parts of the mouth, or the differences in the cirri, of much service; and it will be shown under [T. porosa], that the relative numbers of the segments in the several cirri, and even their shape, is extraordinarily variable.
Affinities.—All the species are pretty closely allied, and there is no ground for making any sectional division of the eight species, more especially not on the ground whether or no the radii are developed. [Tetraclita purpurascens], taking into account all the characters, including the animal’s body, is perhaps the most distinct species in the genus, but it is closely allied to [T. costata]. In the well-marked character of the parietes being formed of a single row of large tubes, [T. rosea] differs from all the other species. The genus is closely allied to [Balanus]; I can point out no difference in the animal’s body, nor any constant difference in the opercular valves. The ridges and crests on the under sides of the scuta are more prominent in [Tetraclita]; and all the species, excepting two, have crests (though sometimes very slight) for the attachment of the rostral depressor muscle, and these occur only in two species of [Balanus]: crests, also, for the attachment of the lateral depressor muscle are common in [Tetraclita], but rare in [Balanus]. The main character, however, of the genus, as compared with [Balanus], is derived from the existence of only four compartments, and in a lesser degree from the several rows of parietal pores; but, as just stated, [T. rosea] has only a single row, and, on the other hand, in [Balanus crenatus], there is a slight tendency exhibited, in the dividing of the longitudinal septa near the outer lamina, to form a second row of parietal pores; and in [B. cariosus], moreover, we actually have two or three rows of irregular and very short pores. The thin, yet solid calcareous basis which occurs in all the species of [Tetraclita], excepting [T. purpurascens], resembles the basis in [Balanus flosculus] and [imperator], but I suspect that the structure of the cement-ducts is different in the two genera.
1. [TETRACLITA] POROSA. Pl. [10], fig. [1 a]-[1 m].
LEPAS POROSA. Gmelin. Syst. Naturæ, 1789.[107]
BALANUS SQUAMOSUS. Bruguière. Encyclop. Method., 1789, Pl. 165, fig. 9, 10.
LEPAS FUNGITES. Spengler. Skrivter af Naturhist. Selskabet, 1 B., 1790.
---- POROSA. Wood’s General Conchology, Pl. 9, fig. 4, 1815.