Cerithium (Triforis) bigemma Watson, 1880

Albano, Paolo G., Bakker, Piet A. J. & Sabelli, Bruno, 2019, Annotated catalogue of the types of Triphoridae (Mollusca, Gastropoda) in the Natural History Museum of the United Kingdom, London, Zoosystematics and Evolution 95 (1), pp. 161-308 : 161

Cerithium (Triforis) bigemma Watson, 1880

Cerithium (Triforis) bigemma Watson 1880: 101-102, not illustrated. Illustration available in Watson (1886): 562-563, pl. XLIII, fig. 6.

Type locality.

"Lat. 18°38'30"N, long. 65°5'30"W, St. Thomas, North of Culebra Island, Danish West Indies" (Puerto Rico).

Type material.

Lectotype: NHMUK 1887.2.9.1762, designated by Rolán and Fernández-Garcés (2008) (not seen) .

Original description.

St. 24. Mar. 25, 1873. Lat. 18°38'30"N., long. 65°5'30"W. St. Thomas, N. of Culebra Island, Danish West Indies. 390 fms. Coral-mud.

Shell.- Sharply conical, high and narrow, solid, opaque, brilliant, yellowish white. Sculpture. Longitudinals-the whorls are crossed by rows of tubercles with broad and rounded hollows between; of these longitudinal rows there are 17 to 18 on the last, and about 14 on a great many of the preceding whorls; besides these the surface is sharply, distinctly, and pretty closely scored by minute lines of growth. Spirals-a prominent spiral band encircles the whorls formed by two rows of rounded tubercles, which in each row are connected by a spiral thread; of these threads the lower is rather the larger, sharper, and more prominent. The distance between these threads is very nearly the same as that between the longitudinal rows, so that each group of four adjoining tubercles forms nearly a rhomb. Round the upper part of each whorl is an impressed flat surface, in which, very near the suture, lies another smaller spiral, which becomes minutely tubercled where it crosses the longitudinal rows. At the bottom of each whorl is a very minute spiral thread, which forms a pouting edge to the suture. Besides these the surface is faintly reticulated by microscopic spirals crossing the longitudinal lines of growth. This reticulation is best seen on the flat and glossy base, which is unbroken except by a small sharp spiral, about 0.012 in. within the edge. Colour yellowish white, pure white on the upper part of the spire; round the base of each whorl is a suffused pale tint of brown, which is more or less the colour of the base of the shell; the point of the pillar is white. Spire high, narrow, and conical, slightly slewed to the left; so that while the left slope is straight, almost concave, the right slope is just perceptibly convex. Whorls probably 22, but of these the 3 or 4 apical ones are broken off; they are of very slow increase, flat, constricted on their upper part, flatly prominent in the middle, and contracted at the lower part; the base of the shell is flatly conical. Suture strongly defined by the depression in which it lies, but itself linear and projecting, being minutely marginated both above and below. Mouth squarely oval, pointed above and at the front of the pillar by the canal, which is small. Outer lip broken. Pillar short, small, straight, scarcely excavated or twisted, at the point sharp and slightly advancing outwards. Inner lip. A very thin layer of glaze is carried across the body, and turns round the pillar in a few microscopic lines, by which alone it can be traced. H. 0.6. B. 0.12. Penultimate whorl 0.072. Mouth, length 0.08, breadth 0.06.

This has a good deal the proportions of C. metula, Lov., with a narrower base. It slightly resembles the Triforis Pfeifferi, Crosse, and (apparently, for the B. M. tablet has more than one species on it) the T. scitula, A. Ad., both from S. Australia; but these have only one series of gemmules, the upper row being very much smaller, and in both the whole shell is very much smaller and slenderer. T. gigas, Hinds, is a much thinner and less strongly tubercled and sutured shell. T. angustissima, Desh. (Moll. de Bourbon), is larger, broader in proportion, has the lower (in his description “supérieure”, as he reverses the shell) row of tubercles larger, and lacks the infrasutural flat constriction with its small and finely tubercled spiral.

Remarks.

Rolán and Fernández-Garcés (2008) recently treated this species and illustrated the lectotype.