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Cratis progressa Hedley, 1915

Description: Shell minute, thick and solid. Prodissoconch a concave semicircular disc with raised centre. Shape subovate, umbones at anterior end of hinge. Anterior and of shell truncate, posterior end smoothly rounded; ventral margin smoothly rounded. Hinge line straight with 2 series of microscopic elongate denticles on either side of a triangular resilifer; large teeth below, 3 on posterior side, 2 on anterior. Interior smooth apart from poorly defined muscle scars; margin with about 5 denticles on posterior-ventral margin and a similar number on antero-ventral margin. Exterior strongly cancellate, formed from dense radial ribs crossed by concentric ribs of about same strength and spacing. Periostracum not seen. Shell colour white or fawn.

Size: Up to 3 mm in length

Distribution: Known from Cape Byron, NSW, southwards to Port Macquarie, NSW, and one lot from east of Sydney in 192-203 m.

Habitat: Subtidal, 47-203 m depth.

Remarks: Another species of Cratis from NSW is Cratis delicata Bergmans, 1970. This species was named from specimens sorted from shell sand from Narrabeen Beach, NSW, but the Australian Museum holds no specimens. It was described as being 1.0 mm in length, and differing from C. progressa by having a much smaller prodissoconch, in exterior sculpture, and in the absence of interior marginal crenulations.

Fig. 1: SYNTYPES NE of Port Macquarie, NSW, in 183 m (C.037760)


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