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.