Caecum hyalinum Vannozzi, 2022

(Figs 3A; 4F)

Caecum hyalinum Vannozzi, 2022: 89, fig. 3A-R.

Caecum cf. gulosum Hedley, 1899 — Vannozzi et al. 2015: 115, fig. 25F.

Caecum arabicum Issel, 1869 — Blatterer 2019: 251, pl. 123, fig. 1a, b.

TYPE MATERIAL. — Holotype. Tanzania • dd; Zanzibar, Uroa Bay; 25-30 m depth; MCZR-M-TYPE-00110.

TYPE LOCALITY. — Indian Ocean, Tanzania, Zanzibar, Uroa Bay, 25-30 m depth.

MATERIAL EXAMINED. — Mayotte • 1 dd worn; Mozambique Channel, W Mayotte, BIOMAGLO Stn DW4847; 12°41’S, 44°57’E; 738-763 m depth; 30.I.2017; MNHN .

DISTRIBUTION. — Whole eastern coast of Africa, from northern Red Sea to northeastern South Africa (Vannozzi et al. 2015; Blatterer 2019; Vannozzi 2022).

DIAGNOSIS. — Shell of average size for the genus, colourless, semitransparent. Tube cylindrical, tapering toward the apex, smooth or with minute rounded rings. No macula. Aperture with a slight but clear swelling. Septum hemispherical, dome shaped with a broad and low nail-like mucro, oriented between the dorsal and the right side. Length 2.47 mm.

REMARKS A single, worn shell was found in the examined material. Although the specimen is slightly larger compared to the holotype and despite the poor condition of this specimen and the incomplete aperture, the shape the septum and the presence of minute rounded rings support its identification as Caecum hyalinum .

This elusive species is known from few specimens so far. It can be compared with C. inflatum de Folin, 1869 and C. armoricum de Folin, 1869, both sharing a hemispherical protruding septum with nail-like mucro. Caecum inflatum from the southwest Pacific is smaller, shows a stouter tube, a narrower and more protruding septum and a simple aperture without swelling. Caecum armoricum from the eastern Atlantic and the Mediterranean Sea has a septum of very similar shape, but possesses a smooth tube without rings and never has a swelling before the aperture.