Animal’s body unknown.
Affinities.—This species is rather more closely allied to the last than to any other. There is a close analogy in the peculiar manner in which the scuta and terga are articulated together in the two species: in this species it is effected by the great development of the articular ridge of the tergum, and in [T. cœrulescens] by that of the scutum. The internally striated calcareous basis, and the internal tubular interspaces between the denticulated ridges of the radii, are peculiar characters. The white colour, the narrowly and closely ribbed parietes, and the broad radii, give this species an aspect, by which it can be easily recognised.
4. Genus—ELMINIUS.
ELMINIUS. Leach. Zoological Journal, vol. 2, July, 1825.
Compartments four: parietes not porose. Basis membranous.
Distribution, Southern temperate seas.
General Appearance.—Shell conical, with a strong tendency in most of the species to become cylindrical: orifice generally large. Walls either thin and smooth, or thick and plicated longitudinally. Colours various, pale purple, greenish, white, and, in [E. plicatus], owing to the exposure of an intermediate lamina of shell, bright orange-yellow. Radii, either of considerable width, with their summits oblique and rounded, as in the first two species of the genus, or very narrow, as in the last two species. [Elminius plicatus] is the largest species, and is sometimes one inch in basal diameter. The outer surface of this latter species is occasionally much corroded.
Scuta: these are of the usual shape; in [E. Kingii] and [modestus] there is no adductor ridge and no crests for the depressor muscles; in [E. plicatus] and [simplex], on the other hand, there is a well developed adductor ridge and crests for the lateral depressor muscles; in some individuals, also, of [E. plicatus] there are small crests for the rostral depressores.
The Terga are remarkable for their variability in all the species; in many specimens of [E. Kingii] and [modestus] the basal margin on the carinal side of the spur is deeply hollowed out. The width and acumination of the spur varies in all the species. In [E. plicatus] and [simplex] this valve is remarkably like that of [Tetraclita porosa]. In some specimens of [E. Kingii] the terga and scuta are firmly calcified together.