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.
_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.