3. [ELMINIUS] PLICATUS. Pl. [12], fig. [2 a]-[2 f].
ELMINIUS PLICATUS. J. E. Gray. Appendix to Dieffenbach’s Travels in New Zealand, p. 269, 1843.
Shell deeply folded longitudinally, corroded, coloured in parts orange: radii very narrow, with their edges sinuous, and slightly dentated: scutum having an adductor ridge.
Hab.—New Zealand; New South Wales(?). Attached to rocks, often coated by [Chamæsipho columna]; Mus. Brit. and Cuming.
General Appearance.—Shell tubulo-conical, or conical, rarely depressed; strong, rugged, coloured in parts bright orange; deeply plicated longitudinally, but with the upper parts corroded and smooth. Orifice large. The sutures are indistinct and almost obliterated; the radii, when most developed, are narrow. Some specimens have their whole surface deeply corroded; in which case they are finely striated longitudinally, or pitted, and are of a gray or brown colour. The largest specimens are one inch in basal diameter, but one depressed specimen was 1.3 in diameter; another was rather under one inch in diameter, and one inch in height.
Scuta; beginning with the common tubulo-conical and not much corroded specimens, the valve (fig. [2 c]) is moderately elongated, but in a rather variable degree. A prominent adductor ridge runs, from a little above a middle point of the basal margin, along the slightly prominent articular ridge: the articular furrow is moderately wide. There are distinct crests for the lateral depressores. In the conical, corroded specimens, the scuta (fig. [2 e]) are considerably broader, with the articular ridge much more prominent, and the furrow wider: in one such specimen, there were crests for the rostral depressor muscle.
The Terga, in the commoner variety, resemble those of [Tetraclita porosa]; the spur adjoins the basi-scutal angle of the valve: the articular ridge is moderately prominent, and the furrow moderately deep. The valve is beaked, with an unusually large internal tube for the thread of corium: the beak, however, is often worn away. In the depressed much corroded specimens, the terga (fig. [2 f]), like the scuta, are broader and shorter than in the commoner variety; and the spur more especially is broader. The scutal margin is much more widely inflected, and the articular ridge much more prominent; consequently the articular furrow is much deeper.
Structure of the parietes and radii.—The orange or yolk-of-egg colour, which is so conspicuous a character in the present species, is due to a layer of shell between the inner and outer lamina, and is exposed only by the corrosion of the latter. Hence the very base of the shell is not of this colour; nor are the uppermost and still more deeply corroded portions, for here the orange-coloured layer has been removed. The sheath is orange-coloured, and the operculum, to a certain extent, is similarly tinted. The epidermis on the parietes, where preserved quite close to the basis, supports remarkably strong spines, about 1/100th of an inch in length. The basal internal edges of the walls are rather coarsely striated with irregular short ridges and sub-cylindrical points; and the walls in most of the specimens are regularly and deeply folded, which, with the little ridges, gives the appearance represented in fig. [2 b], Pl. [12]. I have stated, under the Genus, that in the corroded and depressed specimens, the walls are rendered extremely thick by the inward production and upward extension of these same ridges and points; the under surface of the shell acquiring almost the appearance of [Chelonobia caretta]. The Radii are often not developed, even the sutures being obscure; when most developed, they are narrow, with the outer lamina along the growing edge sinuous, giving to the sutures a crenated appearance. The sinuosities on the growing edge generally send inwards short ridges or septa, like those on the sutural edges of the radii in most [Balanidæ], but of which there is no trace in the other species of [Elminius]. In very minute, colourless specimens, about the 1/20th of an inch in diameter, the radii are quite smooth-edged. The alæ have their edges strongly crenated. The lower edge of the sheath depends freely.
Mouth: the labrum shows some tendency to be bullate; the notch is broad and shallow: the palpi have a thick brush of bristles on their inner sides. The mandibles have four or five teeth. In the maxillæ, the upper spines above the broad notch, are very strong. In the outer maxillæ, the two lobes are widely separated.
Cirri: in the first pair, one ramus is about one fifth longer than the other. In the third pair, the posterior ramus is one fourth longer than the anterior ramus, and its terminal segments are tapering, each having a single circle of bristles: the other segments, and those of the shorter ramus, support many coarsely pectinated spines. In the sixth cirrus, the segments are protuberant in front, and carry four pairs of stout spines, with a tuft of fine bristles between them.