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.