The moveable tergum is broad and rhomboidal. Externally a prominent axial ridge (′′′ in T, in fig. [1 b], &c.), which widens downwards, runs from the apex of the valve to the basal point, and there projecting slightly, causes the scutum to be indented; this indentation on the scutum appears like a third articular ridge, lying beneath (′′), S, in fig. [1 b], &c. Above the lower and axial ridge, on the scutal margin of the tergum, there is a middle articular ridge, which locks in, between the lower (′′) and upper articular ridges (′) of the scutum (S). Again above the middle ridge there is an upper and third articular ridge (′), which is either quite distinct, as in fig. [5], T, or more commonly is formed by the occludent margin of the valve, as in T, fig. [1 b]. The broad extremity of this upper articular ridge is often produced into a slight projection, or shoulder, and this always underlies the scutum, of which the under and upper surface is indented or furrowed (see fig. [1 f]), in order to receive this shoulder. The upper articular ridge of the scutum (′, S, [1 b]) locks in between the upper articular ridge or occludent margin (′, T), and the middle ridge (′′, T) of the tergum.
Hence, altogether, there are three articular ridges on the scutal margin of the tergum, the occludent margin being generally counted as one; whereas, on the tergal margin of the scutum, there are only two ridges, though, as before noticed, an outer indentation, which is developed as a ridge in [V. nexa] (fig. [5], S), might almost be counted as a third articular ridge.
I may here just remark, that the furrow between the two ridges on the tergal margin of the scutum, resembles the articular furrow in the scutum of the [Balanidæ]; but it may be doubted whether the resemblance be more than superficial, as this furrow, in the case of [Balanidæ], receives the edge itself of the tergum, whereas here it receives only a ridge, proceeding from the apex of the tergum, to a nearly middle point on its scutal margin. Finally, I may add, that the tergum in this genus, in general shape, in growth (presently to be referred to), in the manner in which the upper scutal shoulder is overlapped by the scutum, and in the presence of the axial ridge, presents a very striking resemblance to certain old fossil species of Pollicipes, and to a limited extent to the living species of Lithotrya.
The scutum and tergum being interlocked, move together; they can be firmly shut by the contraction of the long adductor scutorum muscle. Their opening appears partly due to the elasticity of the membranous hinge (representing the opercular membrane), by which they are attached transversely, just beneath the summit of the carina and rostrum. No doubt the protrusion of the cirri effectively aids the act of opening. These valves are not capable (nor, of course, the other valves) of any other movement; for there are no muscles for such movements.
Fixed Scutum and Tergum.—The fixed scutum is larger than the fixed tergum, and therefore has the same proportions as the homologous valves in ordinary cirripedia, but reversed proportions compared with the moveable scutum and tergum. The shape of neither valve can hardly be described. The fixed Scutum (S′ in all the figs.), externally, seems at first to consist of two portions, namely, a curved occludent rim (a), closely resembling the opposed occludent margin (a) of the moveable scutum (S), having in fact undergone very little modification, (as may be best seen in Pl. [21], fig. [1 b]); and secondly, of a much modified portion (marked ′′), which resembles in outline and state of surface the rest of the walls of the shell, and may be called the parietal portion. The rostrum (A) curls round the end of the occludent portion, under an edge (b), evidently answering to the basal margin (b) of the moveable scutum, and is simply united to this portion by membrane, but beyond this part, it is articulated to the parietal portion (′′) of the fixed scutum, by oblique interlocking ridges, like those forming the suture between the rostrum and carina. In [V. nexa], however, (fig. [5]) the rostrum (A) does not curl round any part of the interlocking fixed scutum (S′), but articulates with it by a straight suture. Internally, the fixed scutum (S′ in figs. [1 e], [1 c], of reversed shells) has a surprisingly large, thin adductor plate (m in fig. [1 c], [1 b]), with a rounded outline, projecting nearly parallel to the basis or surface of attachment; the adductor scutorum muscle is attached to its upper surface, and consequently the animal’s body lies between this plate and the moveable scutum. In the fixed scutum of [V. nexa], however, there is a deep pit, instead of a plate, for this muscle.
The fixed Tergum, likewise, consists of two portions—a middle and lower, or parietal portion, and a rim or upper portion; the rim consists of two unequal arms, answering to the two upper margins (not merely the edges) of the rhomboidal moveable tergum; the longer rim (x in T′, see fig. [5]) answers to the carinal margin (x) of the moveable tergum, and may be called the carinal rim; and the shorter rim (o and ′ in fig. [5]) answers to the occludent margin (o and ′) of the moveable valve, and may be called the occludent rim. The carina curls round the end of the carinal rim, under an edge, z (much foreshortened in T′ in fig. [1 b], and best seen in fig. [5]), answering to at least a large part of the basal margin (z) of the moveable tergum, and interlocks, by a serrated suture, with the edge of the parietal portion of the valve. Internally (fig. [1 e], less plain in [1 c]) there is a transverse ledge, notched in the middle, and sometimes deeply hollow beneath, running across the valve in about the line of the adductor plate of the fixed scutum: this ledge, in fact, marks and is partly caused by, the line of separation between the central or parietal, much modified, and the scarcely modified, upper or rim portion of the valve. The use of this ledge is apparently to give attachment, as does the under side of the adductor plate of the fixed scutum, to ligamentous fibres, presently to be mentioned, by which the shell is attached to the basal membrane: the carina and rostrum being so much more gently inclined, do not stand in need of a ledge for their attachment.
By comparing the moveable scutum and tergum with the corresponding fixed valves, in all the species, the modification of the latter may be clearly made out to have been effected as follows; and the case appears to me a striking and interesting one. The moveable scutum and tergum lie in the same plane, and are articulated by the means of three ridges on the tergum (including the occludent margin), and by two on the scutum. The fixed scutum and tergum have to be curved, and to be greatly increased in size; and this is brought about, as we shall see, by the large development of a certain small portion of each valve. Comparing first the moveable tergum (T) with the fixed tergum (T′), the umbo of growth matches the umbo of the four margins of the moveable valve, the carinal (x), basal (z), and occludent (o in fig. [5]), margins can be identified with certainty in the fixed valve, from their close similarity in shape, their absolute apposition, or correspondence in position. There remains only the scutal or articular margin, with its three articular ridges; of these, the uppermost (′), inasmuch as in most of the species it is hardly distinct from the occludent margin, can, as we have just seen, be clearly identified, and is overlapped, as it normally should be, by the upper tergal corner of the fixed scutum: the second or middle articular ridge, though not so distinct as in the moveable valves, can be plainly recognised (′′), T′, in fig. [1 b], and [5]; and it serves its normal function of articulating the two valves together. But when we look in the fixed valve for the third or axial ridge (′′′), we find in its exact place, namely, extending from the umbo to the extreme opposite end of the valve, between the second articular ridge (′′) and the basal margin (z, see fig. [5]), only that portion of the valve which I have called the parietal portion; consequently, I do not doubt that this really is the axial ridge largely expanded. So again in comparing the moveable scutum (S) with the fixed scutum (S′); two of the three margins of the former, namely, the occludent (a, see fig. [1 b)] and basal (b), can be identified without a doubt in the fixed valve: the third and tergal margin remains; this should have two articular ridges; of these the upper one, still serving its normal function, can be detected in all the species (′ in fig. [1 b]), and can be seen pretty plainly (′ fig. [5]) in [V. nexa]: but of the lower and other articular ridge there is no sign,—excepting indeed the whole parietal portion of the valve, which, from holding an exactly homologous position with the lower articular ridge of the moveable valve, I cannot doubt in this ridge expanded and curiously metamorphosed. Hence, in both fixed scutum and tergum, it is the outermost or lowest of the articular ridges which has been modified and expanded, so as to rest on and be fixed to the surface of attachment. It would appear as if it had resulted from one ridge in each of these valves having been thus used up by expansion (so to express myself), that the suture between the fixed scutum and tergum is more simple than any other suture in the whole shell; and it is owing probably to this straightness, and consequent tendency to weakness, that the valves do not grow along this line, and so do not become separated from each other during growth, as on the three other lines of suture. As it actually is, owing to this suture never being separated, it is even stronger than the others; its edges on the inside (fig. [1 c]), I may add, are a little inflected or prominent.
Rostrum and Carina: these valves differ from each other, only in the former (A) being rather the largest, and in being more plainly articulated with the fixed scutum, than is the carina (B) with the fixed tergum. Their umbones stand in their normal places, at the two ends of the orifice leading into the sack, that is, facing the dorso-ventral longitudinal plane of the animal; but they are very unequally developed on the two sides, and hence they rise very obliquely from the surface of attachment. Their summits are nearly square, which is caused by the continued growth on both sides of the oblique plates or ridges, by which they are articulated with the adjoining valves. These plates strikingly resemble, as already stated, the radii in certain species of [Chthamalus]. Without these articulating plates, the outline of the rostrum and carina would have been triangular, with the apex upwards. In [V. nexa], in which the walls of the shell are almost perpendicular, the rostrum (A, fig. [5]) is very peculiar and patelliformed, with the umbo sub-central: this results from the development of a border at the upper end of the valve. In this same species, the basal edges of the rostrum, carina, fixed scutum and tergum, are rectangularly inflected, so as to form a ledge round the basis, as in the case of some few species of [Chthamalus],—the ledge appearing like part of the real basis. During the growth of the shell, the upper internal ends of the carina and rostrum are either rendered solid, or a ledge is formed on the inside across their summits, hollow beneath, like the sheath of the [Balanidæ], to which solid or hollow ledge the basal margins of the moveable scutum and tergum are attached by a rim of membrane, forming a hinge.
Direction of Growth: Minute Structure of Valves.—The shell grows downwards all round its basal margin. As far as the diametric growth of its upper part is concerned, there may be said to be only three valves, for the fixed scutum and tergum never become, as already stated, separated; on the three other lines of suture, the valves are added to on both sides; and thus the whole upper part of the shell, and the orifice, increases in diameter. The moveable scutum and tergum grow along their basal margins, and along the margins by which they are articulated together; but the scutum in this latter respect, less than the tergum. The summits of the moveable scutum and tergum, during continued growth, become either worn away, or they project freely; in this latter case, an internal ledge is added round the upper end of the fixed scutum and tergum, so as to keep the orifice accurately closed. In [V. nexa] the rostrum, with its sub-central umbo, is anomalous, as already stated, owing to a broad upper internal border growing in a direction almost directly opposed to the basal growth of the moveable opercular valves.
In young specimens, on the apices of both scuta and both terga, and on the carina, but not on the rostrum, primordial valves may be distinguished, resembling the valves, so called, which first appear (p. [129], [Introduction]) after the metamorphosis in, the Lepadidæ. In the [Verrucidæ], however, they are calcareous; and the minute transverse cylinders, of which they appear to be composed, stand further apart, causing the surface of the primordial valve to be marked with little separate circles, instead of by hexagons.