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Megastomia simplex (Angas, 1871)

Description: Shell medium size, moderately wide, up to 8 teleoconch whorls, last whorl half of shell length, spire weakly convex in outline. Protoconch coiled (Type B) of about 2 whorls, set on teleoconch with axis inclined at more than 90 to teleoconch. Teleoconch whorls rounded, smooth. Aperture teardrop-shaped, length twice the width; columella with single moderate strength plait; interior of outer lip sometimes with about 6 spiral riblets, terminating well before aperture. Shell opaque white.

Size: Up to 7 mm in length.

Distribution: Endemic to Australia. Clarence River, NSW, southwards and around southern Australia to south-western WA, including Tas.

Habitat: Empty shells are from beach washup and down to 27 m. Common.

Comparison: This species is very similar to Agatha petterdi. The most distinctive difference is the shape of the columellar plait, as shown in Fig. 2; in M. simplex the plait is fairly small and symmetrical, but in A. petterdi it is strong and flows continuously to the anterior lip of the aperture

Synonymy: Odostomia angasi, Tryon, 1886 (NEW SYNONYM).

This species was first described as Odostomia lactea Angas, 1867, but that name was preoccupied and hence unavailable. In 1886, Tryon provided a replacement name Odostomia angasi, Tryon, 1886, but in the intervening years Angas had described another species Odostomia simplex Angas, 1871. In my view, Odostomia simplex is the same species as Odostomia lactea. Thus Odostomia simplex is the earliest valid name for the species, and Odostomia angasi is a synonym. Laseron (1951) pointed out the similarity beween O. angasi and O. simplex, but he regarded the presence of riblets within the outer lip as definitive of species. But examination of specimens shows that the riblets are not present at all stages of growth, and their presence is not a useful character for defining species..

Fig. 1: Off Sow and Pigs Reef, Port Jackson (C.011948)

Fig. 2: (Left) Megastomia simplex C. 011948. (Right) Agatha petterdi C.340938


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