Structure of the Parietes and Radii.—As in [Tetraclita], the two lateral compartments are necessarily broad. The parietes are never porose, but consist, in appearance, of a single layer of shell. In [E. modestus] the basal internal edges of the parietes are smooth, but in the other species they are striated longitudinally with short ridges, or sometimes with sub-cylindrical projections. In those specimens of [E. plicatus], which have externally suffered much corrosion, the walls have been rendered extremely thick, by the inward production of these ridges or plates; and in this case the ridges are not confined to the basal edges, but extend upwards close to the sheath. The basal surfaces of the walls in these latter specimens resemble those of [Chelonobia], but the walls in that genus have an internal lamina, which here is not the case. The radii are wide in [E. Kingii], and of moderate width in [E. modestus], with their summits oblique and smoothly rounded, and their sutural edges not in the least crenated. In [E. simplex] they are extremely narrow, smooth-edged, and rounded: in [E. plicatus] they are narrow, and in this species alone the sutural edge is sinuous, and sends inwards short ridges or teeth. The alæ, in all the species except this last, are likewise smooth-edged. The lower edge of the sheath depends, more or less freely, in all the species, except in [E. Kingii].
Basis, membranous in all four species. In [E. modestus], the true basal membrane is extremely thin, and is divided into concentric slips: on its inner surface there are attached numerous cement-ducts, varying from 1/3000 to 1/2000 of an inch in diameter, repeatedly trifurcating, rarely forming hexagonal or quadrangular loops, and with the branches placed approximately parallel to each other. Beneath the true basal membrane there is a complicated layer of cement, in the form of a network, or of separate tubes, or in beads and patches. In [E. Kingii], the basal membrane presented a wonderfully complicated appearance, in part due to the cement forming a mass of inosculating fibres; many of these fibres seemed to end in circular discs of cement.
Neither the Mouth or Cirri offer any noticeable generic characters, as distinct from [Balanus] and several other genera. The Branchiæ, in [E. plicatus], are well developed and moderately plicated. In [E. modestus] they are small, not plicated, but with a rounded sinuous margin: in a specimen having a basal diameter of 25/100, the branchiæ in total length were only 4/100 of an inch. At the bottom of the sack I observed some inwardly pointed, tapering filaments, such as occur in [Balanus]. In this same species I measured the ova, which were unusually elongated, being 19/2000 in length; I may add, that the probosciformed penis was actually thrice the length of the animal’s body in some small but mature specimens (with ova), having a shell with a basal diameter of 16/100 of an inch.
Distribution and Habitats.—This genus is remarkable, inasmuch as it is not distributed over the whole globe: three of the species occur very commonly on the shores of New South Wales, Van Diemen’s Land, and New Zealand; not extending, as far as I can judge, much north of Sydney: the fourth species is confined to South America, ranging from the Falkland Islands and Tierra del Fuego, as far north as Chiloe. [Elminius], therefore, appears to be strictly a southern genus. [Elminius Kingii] and [modestus] represent each other on the American and Australian continents; so I believe [E. plicatus], in New Zealand, represents [E. simplex] in New South Wales and Van Diemen’s Land. The species are all attached to tidal rocks and shells. [E. Kingii] is sometimes attached to floating wood. At the Falkland Islands, the last-mentioned species adhered to some rocks, in a running brook of fresh water, at most eighteen inches under high-water mark, so that for the greater part of each tide it was exposed to absolutely fresh water. At Sydney I found [E. modestus] adhering to oysters in a muddy lagoon, almost separated from the sea, and apparently very unfavorable for cirripedes.
Affinities.—This genus can be distinguished from [Tetraclita] only by the four compartments not being porose, and by the basis being always membranous; whereas, in [Tetraclita purpurascens] alone it is membranous. [Elminius Kingii] and [modestus], on the one hand, are closely allied together, as are [E. simplex] and [plicatus] on the other hand. The last-named species, in the characters of its opercular valves and in its shell, comes nearest to [Tetraclita]. In [T. rosea] we have seen that there is only a single row of parietal tubes, and the outer lamina of the shell is strengthened (as, indeed, it is in most of the other species of the genus, and in [Balanus]) by small longitudinal plates or ridges, which are similar and homologous to those on the internal basal edges of the parietes in three of the species of [Elminius]; so that the difference in the structure of the parietes, in [Tetraclita] and [Elminius], is small.
1. [ELMINIUS] KINGII. Pl. [11], fig. [6 a]-[6 e].
ELMINIUS KINGII. J. E. Gray. Zoological Miscellany, p. 13, 1831.
------ LEACHII. King and Broderip. Zoological Journal, vol. 5, 1832-1834, p. 334, and appendix to King and Fitzroy’s Voyages.
------ ------ G. B. Sowerby. Genera of Recent and Fossil Shells, Plate.