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.