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Seila atkinsoni (Tenison-Woods, 1876)

Description: Shell large in size for the family, narrowly conical, spire straight in outline. Protoconch of 3 whorls, with distinct junction with teleoconch. Teleoconch whorls straight sided; suture indistinct. Whorls sculptured with 4 smooth spiral cords; spirals 1-3 strong, interspaces equal to or greater than cord width; spiral 4 weak, immediately above suture. Axial sculpture restricted to fine, dense axial threads in interspaces. Last whorl with spiral 4 fully exposed, another smooth cord below. Base otherwise smooth, concave. Outer lip of aperture thin; inner lip weakly calloused. Anterior canal a deep U_shaped notch. Shell colour purplish brown.

Size: Up to 14 mm in length, usually much smaller.

Distribution: Endemic to Australia. Port Stephens, NSW, southwards and around southern Australia to at least central SA, including Tasmania. (Number of lots available 25).

Habitat: Intertidal and shallow subtidal, down to at least 100 m. Moderately common.

Comparison: This is the only common species of Seila in NSW. It is narrow, purplish brown in colour, with a moderately tall 3 whorled protoconch. The other species of the genus are:

Seila halligani: Deep water, known only from 2 specimens; white, with protoconch whorls strongly convex.

Seila crocea: Very large (up to 30 mm), orange, moderately common. The protoconch is twice as wide as the other large species, S. atkinsoni.

Seila tenuis: Narrow like S. atkinsoni, but rare. The spire is slightly convex and the protoconch is shorter.

Seila maculosa and Seila nigrofusca. Broad shells with a fairly short protoconch. Both rare.

Synonymy: Cerithiopsis albosutura Tenison-Woods 1877; Cerithiopsis purpurea Angas, 1877. Early literature treated this species as a synonym of Cerithiopsis crocea Angas, 1871, but here C. crocea is regarded as the large orange species of up to 30 mm in length, and C. atkinsoni is certainly not conspecific.

Remarks: Laseron (1951) was unable to confidently identify this slender, purplish brown shell found moderately commonly in Sydney Harbour, but listed it under Seila albosutura Tenison-Woods. The Australian Museum collection now holds sufficient specimens, including specimens from NSW and Tasmania, to make it apparent that the NSW shells are of the same species as those from Tasmania, for which the earliest name is Cerithiopsis atkinsoni Tenison-Woods, 1876.

Figs. 1, 2:  N. of Three Hummock Island, Bass Strait, 51 m. (C.506014)

Copyright Des Beechey 2016