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Vexitomina coxi (Angus, 1867)

Description: Whorls concave above periphery, straight or slightly concave below. Axial sculpture of solid oblique ribs from shoulder to lower suture, 12 or 13 on penultimate whorl. Spiral sculpture of 10-20 spiral cords per whorl, the strongest 2 or 3 creating weak spirally elongate nodules when crossing axial ribs. Body whorl with about 4 spiral rows of nodules . Columella smooth, with callus pad at upper end; outer lip thin, with rounded triangular shaped sinus adjoining suture. Colour fawn, sometimes with irregular dark brown patches below suture.

Size: Up to 33 mm in length, typically about 20-25 mm.

Distribution: Endemic to Australia; Fraser Is, Queensland, to Victoria and Tasmania.

Habitat: Taken in 9-278 m, but most specimens from shallower than 150 m. Moderately common.

Comparison: This species is very similar in shape and sculpture to Vexitomina torquata, but can be distinguished by three features;

Axial ribs: Vertical in V. torquata, usually oblique in V. coxi.

Size: Maximum 16 mm for V. torquata; maximum 32 mm for V. coxi, typically 20-25 mm.

Sinus: Described as "rounded triangular" in V. coxi, U shaped in V. torquata. While there is much variation, usually the upper edge of the sinus is at 45 degrees to the suture in V. coxi, but more parallel in V. torquata.

Callus pad: In V. coxi, elongate and on the parietal wall (see Fig. 1); approximately rounded, under an overhand of the outer lip in V. torquata.

Synonymy: Vexitomina garrardi Laseron, 1954 (NEW SYNONYM)

Remarks: V. coxi varies considerably in shell width and in strength of sculpture. Vexitomina garrardi Laseron, 1954 has been applied to a broad form with strong sculpture (Holotype, Fig. 2). A narrow, deep water form from off Stradbroke Island is shown in Fig. 3.

The shape of the sinus and the strength of the callus pad changes as the shell matures. Fig. 4 shows the changes, from a juvenile shell with no callus pad on the left, to a mature specimen with strong callus pad on the right.

Fig. 1:  5.6 km east of North Head, NSW, in 66 m (C.169544)

Fig. 2:  HOLOTYPE of Vexitomina garrardi; Twofold Bay, NSW (C.103506)

Fig. 3:  Off Point Lookout, Stradbroke Is., Queensland, in 234 m (C.396713)

Fig. 4:  Development of callus pad and sinus with maturity (C.unregistered)

 

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