4. [VERRUCA] NEXA. Pl. [21], fig. [5].
Shell reddish: moveable scutum, with three strongly prominent longitudinal ridges, besides the articular ridges: fixed scutum larger than the carina, with no distinct adductor plate.
Hab.—West Indies, Mus. Brit.; attached to a Gorgonia.
This species differs considerably from all the others in the genus. The shell is brownish-red, tinted yellow: it is not at all depressed like the former species, but the walls are almost perpendicular or even overhang their bases, and the summit of the shell consequently is broad. This form may be in part, but only in part, due to the attachment on the thin branches of the Gorgonia. The umbones of the compartments are remarkably prominent and sharp. Although the parietes are nearly smooth, yet from being so steep, they are little seen, and owing to the very prominent but rounded ribs by which the compartments and opercular valves are articulated together, the whole shell has a strongly ribbed appearance. The diameter of the largest specimen was .2 of an inch.
The rostrum (A, fig. [5]) is patelliformed, with the umbo of growth sub-central, but rather above the middle point; hence this valve, differently from the carina, and differently from the rostrum of the other species, grows not only at its basal margin, and on both sides where opposed to the carina and fixed scutum, but also along its upper margin where opposed to the basal edges of the moveable scutum and tergum: owing to the perpendicularity of this valve, the upper part forms a ledge almost parallel to the orifice of the shell. The carina (B) is of unusually small size, being about only half the size of the rostrum, and scarcely exceeding in size the fixed tergum. The fixed scutum (S′) is large, larger even than the carina; it is oblong, and its shape is more simple than in the other species; this is chiefly owing to the rostrum articulating with the whole of that margin (b) which answers to the basal margin of the moveable valve; whereas in the other species (fig. [1 b]) it curls beyond this margin, and articulates with the very protuberant, so-called, parietal portion of the valve. Three or four rounded prominent longitudinal ribs, exactly like the homologous ribs on the moveable scutum, run from the apex of the fixed scutum to the basal margin, and their extremities form the teeth by which it articulates, as just stated, with the rostrum. Its upper articular ridge (′) is more prominent, and placed much lower down in the suture between it and the fixed tergum, than in the foregoing species. The ledge (o) by which the orifice is kept neatly closed, is here more distinct than in [V. Strömia]: this ledge is necessary, as well as in the case of the fixed tergum, owing to the altered shape of the summits of the moveable scutum and tergum, due to their corrosion and to their coming to project freely. But the most remarkable character of the fixed scutum is, that on the under side there is no great adductor plate, but a rounded hollow with its lower edge only slightly prominent; the absence of the adductor plate, which is present in all the other species of the genus, is no doubt due to the under side of this valve being inclined even outwards, and so standing in some degree opposed to the moveable valve; thus affording on its under surface a place for the attachment of the lower end of the adductor scutorum muscle; whereas in the other species this muscle could not possibly have been attached, without the aid of an adductor plate, to the under side of the much depressed and sloping fixed valve. The fixed tergum (T′) is a little more simple in form than the corresponding valve in the other species; the two arms, answering to the occludent and carinal margins of the moveable tergum, are more nearly equal in length: the internal transverse ledge, separating these rims or margins from the parietal portion of the valve, is but little developed.
All four valves forming the shell are remarkable from having, when full-grown, but not whilst young, their basal edges abruptly inflected inwards, thus forming a ledge all round the basal membrane, as in [Chthamalus intertextus] and [Hembeli].
Moveable Scutum.—This is slightly larger in proportion to the tergum than in the foregoing species: it is chiefly remarkable from the presence of three prominent longitudinal ridges on the main part of the valve, like the two articular ridges on the tergal margin; of these latter, the lower one extends down to about the middle of the tergal margin. The moveable tergum is rhomboidal, with the whole carinal portion marked only by lines of growth: it is only remarkable by the upper of the three articular ridges on the scutal margin being unusually distinct from the occludent margin.
With respect to the animal’s body, its several peculiarities have already been pointed out under the genus. The labrum is decidedly bullate, triangular in section, with a row of minute bead-like teeth on the crest; the palpi are very narrow and short, and do not nearly touch each other: this variation in the structure of the labrum and in the size of the palpi, is very remarkable, considering how important, in a classificatory point of view, these parts are in all other Cirripedes. In the mandibles there are either two or three main teeth, with the whole lower part of the organ pectinated with sharp spines. Cirri: the first pair is not short; in the individual examined, the two rami had eleven and twelve segments. In the second pair, the shorter ramus was two thirds of the length of the longer ramus, the segments being in number ten and fifteen; in the arrangement of the spines this second pair resembles its homologue in the three other species. In the third pair, the two rami are very nearly equal in length, having sixteen and eighteen segments; and the segments of the anterior ramus are only a little thicker and more thickly clothed with spines than those of the posterior ramus. The remaining cirri and the caudal appendages are as in the other species.