[118] It is quite possible that this may be the Balanus duploconus of Lamarck, but with such a character as the following, who can recognise a species? “B. testæ parte supremâ univalvi, indivisâ, convexâ: inferiore turbinatâ, non clausâ: aperturâ ellipticâ. L’exemplaire est sans opercule et incomplet.”
Shell moderately convex, nearly smooth: scutum and tergum calcified together without any suture: scutum furnished with a small occludent ledge, with the adductor ridge descending below the basal margin: tergum square without a spur.
Hab.—Singapore and East Indian Archipelago; Mus. Brit., Cuming, Stutchbury; imbedded in two kinds of coral.
Appearance and Structure of Shell and Basis.—The shell is conical, though to a variable degree, and sometimes is much depressed. The surface is smooth, with only traces of narrow approximate ridges. The colour is white, often with a tinge of dark purple. The orifice is oval, and moderately large. The shell and a small portion of the basis usually stand exserted above the coral. The walls are of variable thickness; when thick, the pores, by which they are permeated, are but little apparent; sometimes there is more than a single row of pores. The points of the septa on the basal edge of the shell are small. The internal surface of the shell is smooth. The sheath is closely attached to the walls, and descends nearly to the basis; on each side its lines of growth are slightly upturned. The basis is deeply cup-formed or cylindrical, and in section oval like the shell; it penetrates the coral to a very remarkable depth,—in one instance to three inches. The shelly layer forming it, is thin, finely furrowed, and not permeated by pores. This is the largest species in the genus; one specimen was three quarters of an inch in its longer diameter, and above three inches in length.
The Scuta and Terga are calcified together, without any trace of a suture; the line of junction can be inferred only from the analogy of [P. conjugatum], in which species the valves have a more normal character, and are separated by a slight furrow. It may be seen in the figure (Pl. [13], fig. [1 d]) of the right and left hand opercular valves, viewed from vertically above in their proper relative positions, how abnormal their appearance is, which is partly caused by the spinose occludent ledges, presently to be described, but chiefly from the carinal margins of the two terga not being straight and parallel, as in all other cirripedes, and therefore not meeting each other, as is usual. In other genera, only the upper part of the carinal margin of the two terga can be opened for the exsertion of the cirri, the lower portion being united by membrane; but here, I have little doubt, from the position of attachment of the adductor muscle (fig. [1 c]), so close to and almost on the terga, that the whole length of the carinal margin of the two terga is free or disunited for the protrusion of the cirri. This opening between the two terga evidently cannot be closed, but is probably filled up, and the animal thus protected, by the dorsal surfaces of the curled-up cirri; such, I believe, being likewise the case with some pedunculated cirripedes, as with Conchoderma. The scutum has a large adductor plate, which extends some little way (namely, about one quarter of the height of the valve), below the ordinary basal margin. This latter margin is slightly sinuous, and a little hollowed out towards the tergal corner of the valve. I believe that the ridge, which runs down to the basi-scutal corner of the tergum, though appearing to be part of the scutum, really belongs to the tergum. The adductor scutorum plate is not, as in the last two species, produced into a point at the rostral angle; at the tergal end it blends into the under surface of the tergum. The occludent margin is coarsely toothed. Rather on the under side of this margin and in the upper part, there is a narrow occludent ledge, which extends up beyond the apex of the valve, and thence runs a little way along the carinal margin of the tergum. This ledge is thickly clothed with strong, yellowish-brown spines, and hence appears like a brush. It is remarkable that the cavity for the adductor scutorum muscle is situated almost on the tergum.
The tergum is of large size, and nearly square; it is, in appearance, separated from the scutum by a ridge running up to the apex. The basal margin forms a right angle with the carinal margin, along which latter margin the lines of growth are upturned, and blend into the occludent ledge, which is common to the two valves. There cannot be said to exist any spur, the whole basal margin being almost straight; nevertheless, on close examination, the ridge which in appearance separate the scutum and tergum, may, I think, be safely considered as one side of the spur (which, it should be remembered, has in all ordinary cases a longitudinal furrow or depression), and the other side of the spur is, apparently, very feebly indicated by a slight flexure in the middle of the basal margin. Hence, if the spur had been developed, it would probably have been half as wide as the valve. There are no crests for the tergal depressor muscles.
Affinities.—The present species, with the last two, form an interesting series. [Pyrgoma grande] and [conjugatum], however, are more closely allied to each other than to [P. cancellatum]. In the scutum, the whole valve is least elongated, with the adductor plate least developed, in [P. grande], and most elongated, with the adductor plate most developed, in [P. cancellatum]. In the outline of the tergum the range of shape is quite remarkable; in [P. conjugatum], which stands between the other two species, the spur is rather long, whereas in [P. grande] there is no spur at all—a very unusual circumstance—and in [P. cancellatum], at the other end of this short series, the spur attains a length wholly unparalleled in any other cirripede.
6. [PYRGOMA] MILLEPORÆ.[119] Pl. [13], fig. [2 a]-[2 f].
[119] From external aspect I suspect this species to be the Creusia madreporarum, Leach (?), as given in Chenu, ‘Illust. Conch.,’ Tab. 1, fig. 6. But I feel sure that Leach has nowhere published this name; and it may be observed that Chenu gives it with a mark of doubt. The shell in its imbedded state is only figured; the opercular valves are not given; and no descriptive details are added. Under these circumstances I have not adopted this name; I have, perhaps, been in some degree influenced by the fact that this species, judging from the many specimens examined by me, is never imbedded in madrepores, but exclusively in millepores, so that Chenu or Leach’s specific name of Madreporarum is singularly inappropriate.