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Genus Mytilus Linné, 1758.
Type: M. edulis L.
Subgenus Aulacomya Moerch, 1853.
Type: Mytilus magellanicus Lam.
Mytilus (Aulacomya) maorianus Iredale.
1915. Mytilus maorianus Iredale, Trans. N.Z. Inst., vol. 47, p. 4, 484.
Locality: Titirangi.
A few damaged specimens, some with ribbing much coarser than is usual in the Recent shells.
Mytilus (Aulacomya) willetsi n. sp. (Fig. 15.)
Shell very small for the group, attenuated, well inflated; beaks terminal; dorsal margin short and straight; posterior margin parallel to the anterior one, both relatively long. Sculpture of numerous waved, radial riblets many of which branch, giving a divaricate appearance. The ribs are strongest down the raised middle of the
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disc and are about 2 per mm. at the ventral margin, on the anterior margin they gradualy become finer until they are from 3 to 4 per mm. The hinge is not clearly exposed but there is a ligamental groove along the dorsal margin.
Height 10 mm.; length 10 mm.; inflation (1 valve) 3 mm. (holotype). A paratype is 13 × 13 × 3.5 mm.
Locality: Waikaripi below Wireless Station, Waitangi.