Chapter XXI: Introduction (17)

_Structure of the Shell and Radii._--The walls are thick: the
parietal tubes are rather large and regular; they become solidly
filled up in their upper parts. The sutural edges of the radii are
formed by unusually narrow, sinuous ridges, sending off delicate
denticuli on each side: the interspaces between these ridges are
solidly filled up. The crenated edges of the alæ are rather thick.
Neither the _mouth_ nor the _cirri_ offer any noticeable character;
but I may observe, that, in the mandibles, the third and fourth teeth
are close together; and that, in the three posterior cirri, the tufts
of small spines between the main pairs are small.

_Affinities._--This species is very distinct from the others, with
the exception of the following _T. radiata_, to which it is in
several respects allied. The under side of the scutum, with its great
articular ridge, and the cylindrical tube formed by the union of the
latter with the short adductor ridge, affords the most noticeable
character.

8. TETRACLITA RADIATA. Pl. 11, fig. 5 _a_-5 _d_.

CONIA RADIATA.[112] _De Blainville._ Dict. Sc. Nat., 1816-1830, Pl.
164, fig. 5, 5 _a_, (sine descript.)

---- LYONSII. _G. B. Sowerby._ Genera of Recent and Fossil Shells,
Plate, 1823, (sine descript.)

[112] The synonymy of this species is complicated. De Blainville
gives no description under the article Conia, published in 1818, or
in the vol. published in 1822; but I believe, from the figures of
the opercular valves, that I have correctly identified this species
with his _C. radiata_. Mr. Sowerby gives no description of _C.
Lyonsii_, or any figure of the opercular valves, but his drawing
of the shell is much better than de Blainville's, and I believe
it is the same species. Whether de Blainville's or Sowerby's
plate appeared first I cannot ascertain. In the second edition of
Lamarck, _Conia radiata_ of Blainville is given as a synonym to
_Balanus radiatus_ of that work; but this is quite erroneous. I
may add that if de Blainville's name does not apply to the present
species, it must to _T. cœrulescens_, and as the latter is the
older name it will be permanent. In this case, _T. radiata_ might
be allowed to stand as my own name, considering that Mr. Sowerby's
figure is imperfect and is not accompanied by any description.
At first I thought that the present species might be the _Lepas
mitra_ of Spengler, ('Skrivter af Naturhist. Selskabet,' 1790,
Tab. 6, fig. 5), but the parietes are not described as porose;
and the folds on the walls are too broad; on the other hand, his
description of the opercular valves makes me think this may be the
same species.

_Shell white, with numerous approximate longitudinal ribs: radii broad, with their summits slightly oblique, internally porose: tergum with the articular ridge extraordinarily prominent, with the spur not joined to the basi-scutal angle._

_Hab._--West Indies, adhering to _Balanus eburneus_ and to _Lepas
anserifera_. New South Wales, adhering to _Tetraclita porosa_.
Attached to _Balanus tintinnabulum_, on a ship's bottom from Sumatra;
not rarely attached to _Balanus tintinnabulum_ on ships' bottoms;
Mus. Brit., Stutchbury, and Cuming.

_General Appearance._--Shell white, rather steeply conical, with
numerous, approximate, rather narrow, longitudinal, rounded ribs on
the walls: in a specimen half an inch in diameter, there were from
eight to twelve ribs on each compartment. The outer lamina of shell
seems always well preserved. Orifice rounded, trigonal. Radii white,
smooth, broad, with their summits only slightly oblique. I have
seen one specimen 1.2 in basal diameter, but quarter of an inch is
a common size, and very young specimens are unusually frequent in
collections.

_Scuta_ broad, externally not striated longitudinally. The articular
ridge is prominent, and the furrow deep, but not in so great a
degree as in _T. cœrulescens_. The adductor ridge is only slightly
prominent; it extends upwards only a little way above the lower end
of the articular ridge, and does not form with the latter a cavity.
There are no crests for the rostral depressor muscle, but there is a
little pit, formed by the folding over of the occludent margin.

_Terga._--These valves, when articulated with the scuta (fig. 5 _b_),
project above them to an extraordinary degree, and are separated
from them by a deep, fissure-like hollow, caused by the remarkable
prominence of the articular ridge of the terga. The upper part of
the tergum is not beaked, and does not project freely much above the
sack. The valve is large; externally there is a rounded longitudinal
furrow. The tergal margin is broadly inflected. The articular furrow
is deep, and the articular ridge far more prominent than in any other
sessile cirripede, for it projects, as measured from the outside
surface, more than half the width of the valve; and consequently
the valve, when viewed vertically from above, almost appears as if
formed by the union of three plates, viz., the articular ridge, and
the outside surface on each side of the spur. The spur is of moderate
width, with the corners rounded: it is placed near, but not close to
the basi-scutal angle, so that there is on this side a small portion
of basal margin, forming nearly a straight line with the margin on
the carinal side. In some young specimens, about the tenth of an
inch in diameter, from the West Indies and from New South Wales, the
spur (the position of which I found varied a little in some other
specimens) was placed nearly in the middle of the valve, and very
nearly at right angles to the basal margin; it is possible that these
may be a distinct species, but without larger specimens to judge
from, I think it more probable that this difference in the tergum is
due to variation and youth.

_Structure of Shell and Radii._--The parietal tubes are commonly
elongated in the ray of the circle: the septa are rather thick, and
strongly crenated at their basal edges. The inner lamina of the
walls is strongly ribbed longitudinally. The broad radii have their
sutural edges formed by ridges, with numerous and closely approximate
denticuli: the interspaces between the main ridges are not soon
filled up, and at the bottom, each interspace usually terminates in
a pore or tube; so that the radii are not solid, as in most of the
foregoing species, but porose. The alæ have their edges crenated.

_Basis_, calcareous, of unusual thickness; the inner, or upper
surface, is striated from the centre in rays, corresponding with the
ribs on the inner lamina of the walls. This striated or furrowed
structure in the basis, shows a tendency to its becoming tubular or
porose, as may be inferred from analogous cases in Balanus.

_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.

_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.

_Shell smooth, gray or dirty white: radii broad, smooth-edged: scutum without an adductor ridge; tergum with the spur distinct from the basi-scutal angle: scutum and tergum sometimes calcified together._

_Hab._--Tierra del Fuego, Falkland Islands, Chiloe. Attached to tidal
rocks and sometimes to floating timber; Mus. Brit., Darwin, &c.

_General Appearance._--Shell fragile, either steeply conical with a
large orifice, or sub-cylindrical; surface smooth, grayish or white,
with large portions covered by pale brown epidermis. Radii broad with
their summits oblique, smooth, slightly arched, exhibiting a large
surface of the alæ. The alæ usually have their summits much less
oblique than those of the radii; the portion added during diametric
growth is of a dead white colour. The growth ridges on the scuta
are very little prominent, and are crossed by a very obscure band
of blueish-gray. The largest specimen which I have seen was .8 in
basal diameter, and the longest cylindrical variety .55 of an inch in
height.

The _Scuta_ are remarkable for not having any adductor ridge or
crests for the depressor muscles; the articular ridge is prominent,
but it is short, not extending down half the valve. I have mentioned
under the genus, that in many specimens at the Falkland Islands the
scuta and terga were calcified together.

The _Terga_ are rather small: the basal margin on the carinal side
of the spur is always hollowed out, but to a very variable degree,
as may be seen in the three figures (6 _c_-6 _e_); this margin is
generally dentated with one or two little points; and an inner lamina
of shell sometimes depends beneath the outer lamina, to which the
opercular membrane is attached, as may be seen in the figure (6 _d_)
of the external surface of the valve. The crests for the depressor
muscles are well developed. The tergal margin is broadly inflected,
and the articular ridge prominent, making the articular furrow deep.
The spur is rather narrow, and is either (6 _d_, 6 _e_) bluntly or
sharply pointed. The basal margin on the scutal side of the spur, is
hollowed out, but to a variable depth.

_Structure of the Parietes and Radii._--The parietes are thin; at
their internal basal edges they are finely striated in longitudinal
lines. The radii are solid, with quite smooth edges; they are
generally covered by the epidermis. The sutural edges of the alæ are
likewise smooth, these are added to largely during the diametric
growth; and their summits, as already stated, are much less oblique
than the summits of the radii. The internal surface of the shell
is smooth, and is tinted pale dull purple. The lower edge of the
sheath can hardly be said to be free. The carinal margins of the
compartments project a little inwards.

_Mouth_: the labrum is deeply notched, and supports five little teeth
on each side; the palpi are thickly clothed with spines on their
inner sides; the mandibles have five or only four teeth: the maxillæ
are notched, and the outer maxillæ bilobed.

_Cirri_: the first pair has one ramus nearly twice as long as the
other. The three posterior cirri are elongated, and each segment
supports five or six pairs of long spines, with a few minute
intermediate bristles.

2. ELMINIUS MODESTUS. Pl. 12, fig. 1 _a_-1 _e_.

_Shell folded longitudinally, greenish or white: radii of moderate breadth, smooth edged: scutum without an adductor ridge: tergum narrow, with the spur confluent with the basi-scutal angle._

_Hab._--New South Wales; Van Diemen's Land; New Zealand; very
commonly attached to littoral shells and rocks; associated with
_Balanus trigonus_ and _vestitus_; Mus. Brit., Cuming, Stutchbury,
Darwin.

_General Appearance._--Shell conical, generally rather steep,
occasionally depressed: walls longitudinally folded, sometimes very
deeply, sometimes only to a slight degree: colour dull greenish or
white. Radii of moderate width, with their summits very oblique,
smooth and slightly arched: alæ much exposed, with their summits
less oblique than those of the radii: the portion added to the alæ
during the diametric growth differs much in appearance from the other
portion. The scuta have the growth ridges but little prominent;
they are crossed by a faint longitudinal band of gray. The largest
specimen out of the many which I have seen, was under .4 of an inch
in basal diameter.

_Scuta_, destitute of an adductor ridge and of crests for the
depressor muscles: the articular ridge is moderate; but the
articular furrow is rather wide: the internal occludent margin is
much thickened. The _Terga_ are narrow and small; they are somewhat
variable in shape, caused by the degree to which the basal margin is
hollowed out (fig. 1 _c_-1 _e_), and likewise by the extent to which
the upper end of the valve has been worn away. No spur is apparent,
for it is confluent with the basi-scutal angle of the valve. The
articular ridge is very prominent, and runs down to the basi-scutal
angle; and as the valve in this part is extremely narrow, with the
spur not developed, it here assumes a channelled structure. The
basi-carinal corner of the valve is furnished with rather feeble
crests for the depressor muscles, and in those varieties in which the
basal margin is much hollowed out, this part is remarkably narrow.

_Structure of the Parietes and Radii._--The internal basal edges of
the parietes and the sutural edges of the radii and alæ, are all
smooth. The lower edge of the sheath depends freely. In the green
varieties the colour is most distinct on the internal surface of the
shell. The four compartments separate very easily when the shell has
been ill preserved in spirits, or after a very short immersion in
caustic potash.

_Mouth_, as in _E. Kingii_, excepting that there are only three teeth
on each side of the notch (which is deeper) on the labrum. The cirri
resemble those of _E. Kingii_; the segments in the sixth pair are
equally elongated, and bear five or six pairs of spines.

_Affinities._--This species is closely allied to its South American
representative _E. Kingii_; the differences consist in its smaller
size, often greenish colour, more folded walls, and narrower radii:
the internal basal edges, also, of the parietes are here smooth,
instead of being striated, as in _E. Kingii_. The terga present even
more obvious differences, in their narrowness, channelled under
surface, and in the absence of the spur, or more properly in its
confluence with the basi-scutal angle of the valve.

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.

_Affinities._--This species differs considerably from the first two
of the genus. In several characters it approaches nearer than the
other species to Tetraclita, especially to _T. porosa_;--namely,
in the scutum having an adductor ridge and crests for the lateral
depressores, in the whole form of the tergum, in the thick walls
liable to much corrosion, in the narrow radii, and in their edges,
as well as those of the alæ, being crenated; and, lastly, in the
character of the cirri, more especially of the third pair, with
its coarsely pectinated spines. It also approaches, in all its
characters, _Balanus imperator_ and _flosculus_.

4. ELMINIUS SIMPLEX. Pl. 12, fig. 3.

_Shell ribbed longitudinally, dirty white; radii extremely narrow, smooth-edged; scutum having an adductor ridge._

_Hab._--New South Wales (Sydney and Twofold Bay); Van Diemen's Land;
tidal rocks, often attached to other Cirripedes, and associated
with _Balanus nigrescens_, _Tetraclita purpurascens_, _Catophragmus
polymerus_; Mus. Brit., Cuming, Stutchbury, and Darwin.

This species, of which I have seen specimens from the above three localities, all exactly agreeing with each other, is perhaps the Australian representative of _E. plicatus_, which seems to be confined to New Zealand.[113] In all essential points it comes so near that species, that I shall make the greater part of my description comparative.

[113] I am bound to state that I have seen two specimens of _E.
plicatus_ marked Sydney, and one marked Moreton Bay, but in both
cases the collectors had visited New Zealand, so that a wrong
habitat by mistake might easily have got attached to the specimens
in question.

_General Appearance._--In external appearance there is considerable
difference from _E. plicatus_, for _E. simplex_ is generally of a
regular conical shape, of a dirty-white colour, with the surface well
preserved, having moderately wide, not very prominent longitudinal
ribs. The orifice is rather small and pentagonal. The radii are
extremely narrow or linear, with quite smooth edges; the sutures,
however, are always very distinct, and in the upper part, the alæ
are generally rather widely exposed, as viewed from the outside.
The largest specimen which I have seen was .7 of an inch in basal
diameter.

The _opercular valves_ are closely similar to those of _E. plicatus_,
but the _scutum_ is generally a little more elongated, and the
articular furrow not so deep: in accordance with this last fact,
the articular ridge in the _tergum_ is not so prominent as in _E.
plicatus_; but we have seen that these several characters are highly
variable in _E. plicatus_. The slope of the basal margin of the
tergum towards the spur varies in the present species, in a strictly
analogous manner, as it does in _Tetraclita porosa_.

_Structure of the Parietes and Radii._--The parietes are not so thick
as in _E. plicatus_; internally they are tinted pale purple; when
broken transversely, a row of microscopically minute orange-coloured
dots can generally be distinguished between the outer and inner
laminæ; and these evidently represent the orange-coloured layer in
_E. plicatus_. The sheath also exhibits a faint tinge of orange. The
radii are very narrow, and are quite smooth-edged, differently from
in _E. plicatus_. The edges of the alæ barely exhibit a trace of
being crenated.

In the body I could perceive no difference from _E. plicatus_,
excepting that in the third pair of cirri the two rami are like each
other, and do not support any coarsely pectinated, only serrated,
spines; but after what we have seen on the variability of these very
same characters in _Tetraclita porosa_, I dare not trust to them. The
three posterior pairs of cirri, also, seem here to be more elongated
in proportion to the others, than in _E. plicatus_.

_Affinities._--It is certain that this species is most closely allied
to _E. plicatus_; but as I have seen many specimens of the latter
brought by different persons from New Zealand, and as I have observed
in them no approach to the characters of _E. simplex_, which, in
specimens from three localities, also appear to be constant, I have
considered the two forms as specifically distinct. The present
species differs from _E. plicatus_, in its white, conical, moderately
ribbed, well preserved, smaller shell; and more especially in the
orange-coloured intermediate lamina of _E. plicatus_ being here
represented only by microscopically minute dots. But the radii being
smooth-edged, is the most important differential character, though in
_E. plicatus_, during its earliest growth, whilst still immature and
colourless, the radii are likewise smooth-edged.

5. _Genus_--PYRGOMA.

PYRGOMA. _Leach._ Journal de Physique, tom. 85, 1817.

BOSCIA. _Ferussac._ Dict. Classique d'Hist. Naturelle, 1822.

SAVIGNIUM. _Leach._ Zoological Journal, vol. 2, July, 1825.

MEGATREMA. _Ib._ Ib.

ADNA. _Ib._ Ib.

DARACIA. _J. E. Gray._ Annals of Phil. (new series), August, 1825.

CREUSIA. _De Blainville._ Dict. Sc. Nat., Pl. 116, 1816-30.

NOBIA. _G. B. Sowerby, junr._ Conchological Manual,[114] 1839.

[114] The name, Nobia, is given in this work on the authority of
Leach, but this must be a mistake, probably caused by some MS.
name, (that fertile source of error in nomenclature), having been
used.

_Shell formed of a single piece; basis cup-formed, or sub-cylindrical, attached to corals._

_Distribution_, imbedded in corals, chiefly in the tropical seas,
round the world.

I feel no hesitation in including the above several genera in one genus. In external appearance the _P. monticulariæ_ (Pl. 13, fig. 5 _a_), which forms the genus Daracia of Gray, is the most distinct, but it is so intimately allied to the two, indeed to the three, foregoing species, that it cannot be separated from them. Of the first five species, _P. grande_, _conjugatum_, and _cancellatum_, form a graduated series, but with the steps very distinct, the chief difference being in the length of the spur of the tergum; for the fact of the scutum and tergum being calcified together in _P. grande_ and _conjugatum_, and distinct in _P. cancellatum_, is certainly unimportant, as may be inferred from what we shall see in comparing together the last four species of the genus, and from what we shall see in _Creusia spinulosa_. The three above-mentioned graduated species are connected with the last four species of the genus, by several points of resemblance between _P. grande_ and _crenatum_. The first two species, namely, _P. Anglicum_ and _Stokesii_, are the most closely related together, and may indeed possibly be identical; these two, in all the characters derived from the opercular valves, resemble Balanus and other ordinary forms, and for this very reason they have some claims to be generically separated from the other species of Pyrgoma; for in these latter, the opercular valves seem to have broken loose from all law, presenting a greater diversity in character than do all the other species of Balaninæ and Chthamalinæ taken together.

_General Appearance._--The shell consists of a single piece, generally without any suture, even on the internal surface; and this is the case, at least in _P. Anglicum_, in extremely young colourless specimens: nevertheless, in some specimens of this very species, and of _P. conjugatum_, there were traces of two, but only two, sutures on the sheath, one on each side towards its carinal end. The shell is much depressed or actually flat; and I have seen specimens even slightly concave; in _P. Anglicum_, however, the shell is steeply conical. The outline is generally oval; but in _P. monticulariæ_ it is extremely irregular. The surface is generally furnished with more or less prominent ridges, radiating from the orifice, which is oval and small; sometimes, as in _P. monticulariæ_, excessively small. The colour is white, or pinkish-purple. Most of the species are small, but I have seen specimens of _P. grande_ three quarters of an inch in diameter in the longer axis, and, including the deep, almost tubular, basis, more than three inches in length or depth.

_Opercular Valves._--In three species, viz., _P. conjugatum_, (Pl. 12, fig. 7 _c_), _grande_, and _monticulariæ_, the scuta and terga, on each side, are calcified together so perfectly, that there is no trace of a suture or line of junction: in _P. milleporæ_, these valves are generally slightly calcified together, but with the suture distinct. The _Scuta_ differ so much in shape in the different species, that little can be said of them in common: in _P. Anglicum_ and _Stokesii_ they resemble those of Balanus; but in the other species they are much more elongated than is usual, and this is carried to an extreme in the last four species; this elongation is due to a great increase in breadth, as may be inferred from the position of the apex of the valve, and from the direction of the lines of growth. But the two most remarkable characters are, first,--the extraordinary development of the adductor ridge, so that, in _P. conjugatum_, _cancellatum_ (Pl. 12, fig. 5 _c_), _grande_, and _crenatum_, it extends considerably beneath the basal margin, being produced, in the first two species, at the rostral angle, into a point; at the tergal end of the valve, the adductor ridge, when thus much developed, blends into the articular ridge. The second very remarkable character is the addition of a special ledge along the occludent margin of the scutum, and along the carinal margin of the tergum, which I will call the occludent ledges (_limbus occludens_), and which serve to close the orifice leading into the sack. The occludent ledge is small in _P. grande_, and is clothed with thick yellow spines, giving it a brush-like appearance: in _P. crenatum_ and _dentatum_ it is largely developed, the ledge on the scutum being articulated with that on the tergum, as shown in Pl. 13, fig. 4 _a_, 4 _b_,--the ledges being here and elsewhere marked by little bristly points. In _P. monticulariæ_, however, this ledge arrives at its maximum development (Pl. 13, fig. 5 _f_), the rest of the valve (the scuta and terga being here, as in _P. grande_, calcified together) being reduced to a mere basal edge or border. Excepting for the adductor muscle, the depressions or crests for the other muscles, both on the scuta and terga, are hardly developed.

_Terga_: these, as in the case of the scuta, differ so much in shape in the several species, that little can be said of them in common. In _P. Anglicum_ and _Stokesii_, they are of the normal shape; in _P. cancellatum_ (Pl. 12, fig. 5 _d_) this, to a certain extent, is likewise the case, but the spur is produced to a quite extraordinary length. In _P. grande_ (Pl. 13, fig. 1 _b_) there is no distinct spur, and the whole valve is square. In _P. milleporæ_ (2 _f_) there is no spur, and the valve is arched and triangular. In _P. crenatum_ the spur is broad, rounded, and depressed (fig. 4 _b_), with the carino-basal portion of the valve reduced to a mere border, barely distinguishable from the great occludent ledge. In _P. monticulariæ_ there is no spur, and the whole valve forms a mere border to the occludent ledge; and, lastly, in _P. dentatum_, the valve is extremely variable in shape (fig. 3 _c_, 3 _d_, 3 _f_), and on its internal surface (fig. 3 _g_) there is an inwardly projecting, most singular and anomalous, tooth. Hence we see how wonderfully variable the terga are in this genus.

_Structure of the Walls._--The shell consisting, as has been stated, of a single piece, is generally thick. From the close alliance between this genus and _Creusia_, it is probable that the shell, if examined immediately after the metamorphosis, would be found to show traces of being formed of four compartments. The walls are either solid or porose; their basal margin is formed by strong crenated ridges, answering to the longitudinal septa in Balanus; but these in _P. monticulariæ_ are modified into a very irregular surface. The internal surface of the shell is generally smooth, or only slightly ribbed. The sheath has its lower edge free in _P. Anglicum_ and _Stokesii_, and in a slight degree in _P. milleporæ_, but in the other species it is closely attached to the walls. In _P. monticulariæ_ the sheath might easily be overlooked. In _P. Anglicum_, _grande_, and _crenatum_, it descends almost to the basal margin of the depressed shell, and as the opercular valves and membrane are attached to the lower edge of the sheath, the animal's body necessarily comes to be almost exclusively lodged in the cup-formed basis. In _P. grande_ and _conjugatum_, the lines of growth in the sheath are bent upwards on each side, at points corresponding with the line of union between the scutum and tergum, in a manner I have not seen in any other cirripede; and this sometimes gives the appearance of two lateral sutures. I may here remark, that the manner of growth in Pyrgoma is almost the converse of that in Balanus, Tetraclita, and other allied genera; for in these latter, the basis increases in diameter, and the shell chiefly in height; whilst in Pyrgoma, the shell, from being so flat, increases almost exclusively in diameter, whereas it is the basal cup which is added to in height or depth.

_Basis._--This in all the species is more or less regularly cup-formed or sub-cylindrical. In _P. grande_ it penetrates the coral to a surprising depth; in _P. monticulariæ_ it is irregular in outline, corresponding with the shell. The basis is generally almost wholly imbedded in the coral; but this is not the case with _P. Anglicum_, in which the basis is generally exserted, as it is in a slight degree in _P. grande_. The shelly layer forming the basis, in most of the species, is very thin, and is finely plicated owing to its edge folding between the ridges or septa that form the basal edges of the shell; this is very conspicuous in _P. cancellatum_. The basis is not permeated by pores, except in _P. Anglicum_. In some sessile cirripedes a cleft, covered only by membrane, may be observed all round between the lower edge of the shell and the basis; a cleft of this nature is rather conspicuous in _P. crenatum_, so that small portions of the septa on the internal surface of the walls can be seen from the outside. In _P. monticulariæ_, an analogous structure, developed to an extreme degree, presents a very different and unique appearance; the shell is nearly flat, and the smooth outer lamina does not nearly reach to the circumference, a wide border being thus left exposed, which is roughened (Pl. 13, fig. 5 _a_, 5 _c_, 5 _d_, 5 _e_) by the exposure of the irregular septa. I have not seen a fresh specimen, but there cannot be a doubt that this border is properly covered by membrane.

_Animal's Body._--From some cause, perhaps from the corals in which the species of Pyrgoma are imbedded, long remaining damp, the internal organs are generally badly preserved. I have received, in spirits of wine, only _P. Anglicum_, but I have examined dry specimens, in a tolerable condition, of _P. milleporæ_ and _crenatum_. As neither the mouth nor cirri, in these three species, offer any noticeable characters, distinct from those in Balanus or Acasta, my ignorance of these organs in the other six species is not important. In the above three species the labrum is deeply notched, with about three teeth on each side of the notch, except in _P. milleporæ_, in which the number is six. In all, the mandibles have five teeth, the two lower ones being small: the maxillæ are not notched: the outer maxillæ are bilobed. In the _Cirri_, the rami in the first pair are very unequal in length, the segments being slightly protuberant in the shorter ramus. On the segments of the posterior cirri there are four pairs of spines in _P. Anglicum_, and three pairs (of which the second and third are short) in _P. milleporæ_. At the dorsal basis of the penis there is a small straight projecting point.

_Affinities._--The species (with the exception of the first two) are much more distinct from each other, and more easily determined than is usual with sessile cirripedes; it is, however, quite useless to attempt naming the species without disarticulating and cleaning the opercular valves. Although these valves differ so greatly in some of the species from those of Balanus and the allied forms, the genus itself, as a whole, does not differ much, except in the shell not being divided into compartments, and in the basis being cup-formed and not generally permeated by pores,--these latter characters being in common with Acasta. With respect to the absence of separate compartments, it should be remembered, that in the same species of Tetraclita we have individuals with the four compartments distinct and furnished with radii, and other individuals without any trace of a suture externally,--the outer lamina of shell (though not the inner) having become completely confluent all round. At the commencement of this description, when giving my reasons for uniting the several proposed genera into one genus, I gave a sketch of the affinities of the species: I have only to add, that the following sub-genus Creusia is closely, perhaps too closely, allied to Pyrgoma.

_Geographical and Geological Distribution._--Most of the species are inhabitants of the hot coral-growing zones, in both the eastern and western hemispheres, but more especially, as it would appear, in the East Indian Archipelago. From the habits of the corals, most of the species must be inhabitants of shallow water. _Pyrgoma Anglicum_, however, is an inhabitant of deep water on the southern shores of England, whereas at St. Jago, in the Cape de Verde Islands, I myself collected it adhering to a Caryophyllia, within the tidal limits. This same species existed on the shores of England during the Coralline Crag period; and at this epoch it attained a larger size than at present. Two species of the genus, according to Sismonda, are found in the tertiary beds of Piedmont. Mr. Stutchbury, who is so well acquainted with recent cirripedes, informs me that he has for many years examined fossil Secondary corals, in the expectation of finding imbedded, species of this genus or of some allied form, but without success. The same species of Pyrgoma is by no means always confined to the same coral: I have seen _P. crenatum_ on four or five different corals, and _P. Anglicum_ on at least three kinds: on the other hand, I have seen _P. milleporæ_ only on the _Millepora complanata_ (a member, as I believe, of the vegetable kingdom), and _P. monticulariæ_ on a _Monticularia_ from near Singapore.

1. PYRGOMA ANGLICUM. Pl. 12, fig. 4 _a_-4 _c_.

PYRGOMA ANGLICA. _G. B. Sowerby._ (sine descript.) Genera of Recent
and Fossil Shells, fig. 7, No. 18, Sept. 1823.

MEGATREMA (ADNA) ANGLICA. _J. E. Gray._ Annals of Philosoph. (new
series), vol. x, Aug. 1825.

PYRGOMA SULCATUM. _Philippi._ Enumeratio Molluscorum Siciliæ, Tab.
12, fig. 24, (1836).

---- ANGLICA. _Brown._ Illustrations of Conchology, (2d edit.,
1844), Tab. 53, fig. 27-29.

_Shell steeply conical, purplish red: orifice oval, narrow: basis permeated by pores, generally exserted out of the coral: scutum and tergum sub-triangular._

_Hab._--South coast of England and of Ireland, (12 to 45 fathoms,
Forbes and MacAndrew); Sicily; Madeira; St. Jago, Cape de Verde
Islands; generally attached to the edge of the cup of a Caryophyllia,
in deep water, but at St. Jago within the tidal limits; Mus. Brit.,
Cuming, Lowe, &c.

_Fossil_ in the Coralline Crag, Ramsholt; Mus. S. Wood.

_General Appearance._--Shell steeply conical, slightly compressed,
the lower part with rounded, approximate, radiating ribs: colour
dull purplish-red: orifice oval, small, and narrow. The basis is not
deeply conical, and occasionally is even flat. Generally it stands
exserted; but in the Coralline Crag specimens, it is almost wholly
imbedded. Externally it is furnished with ribs corresponding with
those on the shell. The largest recent specimen which I have seen,
from St. Jago, was .22 of an inch in basal diameter; but some few of
the British specimens are nearly as large, and one of the fossils
from the Coralline Crag a little larger.

The _Scuta_ and _Terga_ are of the ordinary shape of these valves in
Balanus and its allies. _Scuta_ triangular, with the basal margin a
little curved and protuberant: adductor and articular ridges distinct
from each other, moderately prominent: there is a small hollow for
the lateral depressor muscle. _Terga_ triangular, with the spur
rather narrow, moderately long, placed near, but not confluent with
the basi-scutal angle of the valve. The basal margin forms an angle
rather above a right angle with the spur. Internally, the articular
ridge and crests for the depressor muscles, feebly developed.

_Internal Structure of the Shell and Basis._--Internally, the shell
is ribbed more or less prominently. The lower edge of the sheath,
which is reddish, and extends far down the walls, seems always to
project freely. In several specimens there were on each side, at
the carinal end of the shell, a trace of a suture, which could
be perceived only on the sheath. The basis appears always to be
permeated by minute tubes or pores, though these are sometimes rather
difficult to be seen.

2. PYRGOMA STOKESII. Pl. 12, fig. 6.

MEGATREMA STOKESII. _J. E. Gray._ (sine descript. aut figurâ) Annals
of Philosophy, (new series), vol. 10, Aug. 1825.

_Shell moderately conical, pale-purplish red; orifice oval: basis not permeated by pores, deeply imbedded in the coral: scutum and tergum sub-triangular._

_Hab._--Imbedded in the Mycedia (Agaricia) agaricites; therefore from
the West Indies;[115] Brit. Mus. and Stutchbury.

[115] I am greatly indebted to Mr. Dana for having named for me the
coral in which this species was imbedded, and informing me that it
is a West Indian species.

This species comes so close to the last, that I am not sure that I have acted rightly in retaining it, but I think that it is distinct; and in this case, it is the representative, on the other side of the Atlantic, of _P. Anglicum_ of our own side. It will be sufficient to point out the few points of difference. The shell is much more depressed, with the orifice oval, larger, and not so narrow. It is apparently of a paler red, and the radiating ribs perhaps not so prominent. The basis offers the most important difference, being deeply imbedded in the coral; and there is not the least appearance of the thin shelly layer, of which it is composed, being permeated by pores, as, we have seen, is always the case with _P. Anglicum_. As in this latter species, the sheath here depends freely. The opercular valves are closely similar; but in the scutum, the adductor ridge occupies a rather more central position; and in the tergum, the basal margin is more inclined towards, or forms a greater angle with, the spur: these differences, by themselves, I consider quite insufficient to characterise a species; but conjoined with the flatter shell, the larger orifice, the more deeply imbedded and non-porose basis, they may, I think, be admitted as specific. In dimensions, this species seems to attain a slightly larger size than _P. Anglicum_, for several specimens were .22 of an inch in diameter.

3. PYRGOMA CANCELLATUM. Pl. 12, fig. 5 _a_-5 _f_.

PYRGOMA CANCELLATUM. _Leach_ (!). Encyclop. Brit., Supplement, vol.
3, Pl. 57, 1824.

---- LOBATA. _J. E. Gray_ (!). Annals of Philosophy, (new series),
vol. 10, 1825.

CREUSIA RAYONNANTE. _De Blainville._ Dict. Sc. Nat. (sine descript.),
Pl. 116, fig. 7, _a_, _b_.

_Shell with the circumference generally lobed: scutum elongated, with the adductor ridge descending far below the basal margin, and produced at the rostral end into a square point; tergum with the spur four times as long as the upper part of the valve._

_Hab._--Imbedded in a Gemmipora, probably from the West Indies;[116]
Mus. Brit. and Cuming.

[116] In this case I am again indebted to Mr. Dana for naming the
coral; he informs me that the genus is found in the Pacific and
East Indies; the specimen sent he believes is the _G. cinerascens_.

_Appearance and Structure of Shell._--Shell nearly flat, sometimes
tinted dull dirty purple, with the surface marked by slight, broad,
approximate ridges, the ends of which form considerable projections,
giving to the shell, when not too much encrusted by coral, a lobed
border. In young specimens (.15 of an inch in diameter) some of
the points projected half as much as the semi-diameter of the
shell, giving it a radiating appearance. The orifice is oval and
rather small, but of variable size. The shell is thick, and, near
the outer lamina, is penetrated by pores: the internal surface is
smooth. The sheath (fig. 5 _b_) extends down but a short distance
from the orifice; it is closely attached to the walls: the lines of
growth, at a point on each side, bend a little downwards (instead of
upwards, as in the two following species), and hence the lower edge
of the sheath irregularly projects downwards on each side. The basal
cup, internally, is plicated, the hollows corresponding with the
projecting, longitudinal, parietal septa, which form the lobed border
of the shell. The largest specimen which I have seen, was rather
above .4 of an inch in diameter.

The _Scuta_ and _Terga_ are not calcified together: they are both
much elongated. _Scutum_ (fig. 5 _c_, 5 _e_).--For the first time in
the genus, or indeed in the family, this valve presents a remarkable
character in the adductor ridge being immensely developed, so as to
project far below the ordinary basal margin. At the rostral end, it
at first appears to project even more than it really does, for the
toothed occludent margin is in fact a prolongation of the true valve,
as distinct from the adductor plate. Excluding this very narrow,
prolonged, occludent margin, the adductor plate projects for a
length equalling the rest of the valve. Along the tergal margin, the
adductor plate is united to the articular ridge; and at the rostral
end, it is produced into a square tooth, whence a square-edged ridge
extends on the surface of the plate upwards to the ordinary basal
margin of the valve. The exact shape of this adductor plate varies
a little, as does the degree to which it is closely attached to the
ordinary basal edge of the valve. The valve, as distinct from the
adductor plate, is narrow, with the basal margin regularly curved.
The articular ridge is very prominent.

_Terga_ extremely narrow, linear, consisting in chief part of the
spur, which is fully four fifths of the entire length. Externally
(fig. 5 _d_) the valve is furrowed, with the edges more or less
folded in along the spur. The upper or ordinary part of the valve is
about one third wider than the spur. The basi-carinal angle is sharp,
owing to the basal margin being a little hollowed out. A special
plate of shell (fig. 5 _f_), hollow under its basal edge, runs from
the carinal margin to the articular ridge, which latter is situated
in the middle of the valve, and against which the articular ridge of
the scutum abuts. The spur is central; its end is bluntly pointed.
The total length of the tergum rather exceeds that of the scutum, the
produced adductor ridge being included in the latter.

_Affinities._--Under _P. grande_ I shall make a few remarks, showing
that in several characters _P. cancellatum_ and _grande_ are at
opposite ends of a short series, with _P. conjugatum_ intermediate
between them.

4. PYRGOMA CONJUGATUM.[117] Pl. 12, fig. 7 _a_-7 _c_.

[117] Dr. Gray thinks this is the _Pyrgoma stellata_, of Chenu,
('Illust. Conch.'); it may be so; but the figure given of the
shell will do equally well or rather better for the _Pyrgomum
dentatum_ of this work, and for some varieties of _P. crenatum_.
Without a careful description of the opercular valves, it is really
impossible to recognise, with any approach to certainty, the
species of this genus.

_Shell nearly flat with approximate radiating ridges: scutum and tergum calcified together without any suture: scutum with the adductor ridge descending below the basal margin, and produced at the rostral end into a point: tergum with the spur about as large as the upper part of valve._

_Hab._--Red Sea; Brit. Mus. and Cuming.