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Brachidontes rostratus (Dunker, 1857)

Description: Shape equivalve, inequilateral, elongate, with umbo at anterior end; variable in width to height ratio. Dorsal margin convex; postero-dorsal end extended, broadly rounded. Hinge with prominent teeth in small specimens, becoming fused together or obsolete in large specimens; usually 2 in left valve and one in right. Ligament long. Interior scars of posterior pedal retractor muscle and adductor muscle combined to give the appearance of a single elongate scar posteriorly; margin crenulated by external sculpture. Exterior with radial riblets. Colour of exterior black; interior dark purple posteriorly, grading to pale purple anteriorly. Periostracum smooth, black, but shell usually eroded.

Size: Up to 50 mm in length.

Distribution: Sussex Inlet, NSW, southwards to South Australia, including Tasmania.

Habitat: This species lives on exposed rocky shores at mid-tide level and below. Individuals form dense aggregations which causes some distortion of shape of crowded individuals.

Comparison: Brachidontes erosus (Lamarck, 1819) is a similar species which occurs in southern Australia from Victoria to Western Australia. It has sometimes been confused with B. rostratus, but it does not occur in NSW.

Synonymy: Known previously as Austromytilus rostratus (Dunker, 1857).

Remarks: The shell of this species varies in shape depending on the degree of crowding when growing.

Fig. 1: Twofold Bay, NSW (C.118960)

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