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.
Appearance and Structure of Shell.—Shell white, or with a tinge of purple; nearly flat, with moderately prominent, narrow, approximate ridges, radiating from the orifice, which is oval, rather narrow, and not very small. In the largest specimens the ridges are less prominent than in the figure given. The walls are thick, and not at all porose: the sheath extends down almost to the base of the shell, and its lower edge is closely attached to the walls: on each side, towards the carina, there is a trace of a suture, and the lines of growth on the sheath are here a little upturned. The basis is deeply imbedded and internally furrowed; the calcareous layer forming it, is thin. The largest specimen was .4 of an inch in the longer diameter.
The Scuta and Terga (fig. [7 b], [7 c]), in all the specimens which I have seen, are calcified together, with no trace of a suture. There is, however, a slight furrow, which, I believe, marks the normal line of separation between the two valves; and in the following description this is assumed to be the case. The Scutum has the adductor ridge greatly developed, so as to project below the ordinary basal margin to a distance as great as the height of the valve. At the rostral end, this adductor ridge or plate is produced into a point; and at the tergal end, it is blended with the articular ridge, and united to the inner face of the tergum. That portion of the scutum which corresponds with the valve in ordinary cases, and alone is externally visible as long as the operculum is united by the opercular membrane to the sheath, is narrow, with the basal margin considerably hollowed out: the occludent edge is formed into thick teeth. The Tergum is elongated, rather exceeding in length the scutum, the latter being measured from the apex to the rostral projection of the adductor plate. The surface of the valve is depressed in the line of the spur, with the basal end of the latter bluntly pointed. A very slight flexure (fig. [7 c]) on the basal margin indicates where we may believe the spur to commence, showing that it rather exceeds in length the whole upper part of the valve. The lines of growth obscurely indicate a tendency to the formation of a slight “occludent ledge” along the carinal margin. Traces are just visible of crests for the attachment of the tergal depressor muscles.
5. [PYRGOMA] GRANDE. Pl. [13], fig. [1 a]-[1 d].
NOBIA GRANDIS. G. B. Sowerby, junr. (sine descript.) Conchological Manual, fig. 29, 1839.[118]
CREUSIA GRANDIS. Chenu. Illust. Conch., Tab. 1, fig. 2 a, sed non, fig. 2.