Shell large, up to 31.74 mm long, moderately arched (length/height ratio 2.55-2.91, mean 2.72, SD 0.12, n = 7), apex weakly inclined anteriorly. Anterior end occupying 31.0-34.0% (mean 32.1%, SD 1.28, n = 7) of shell length; anterior slope very weakly convex, lateral and posterior slopes broadly convex. Aperture gently arched from side to side. With increasing growth, longitudinal axis gradually twisting roughly 5° anticlockwise between teleoconch I and end of teleoconch II in adults. Teleoconch I depressed conical, length 1.70-2.55 mm (mean 2.03 mm, SD 0.26, n = 7), anterior and posterior slopes flat to weakly concave, lateral slopes flat, glossy, essentially smooth apart from commarginal growth lines and obscure radial lines. Teleoconch II sculptured with strong radial costae and commarginal ridges, rounded nodules at intersections; fine, crisp, crowded commarginal growth lines throughout. Radial costae rounded, crowded, multiplying by intercalation. Radular teeth (adult, holotype MNHN IM-2009-5798) roughly 360 urn long, with 16 cusps, posterior pair fused. Holotype length 26.5 mm.

Pectinodonta gamma n. sp. is closely related to P. beta n. sp. and the two are indistinguishable using the 28S gene but are recovered with COI and ITS1. While the genes indicate that they are closely related allopatric species, regrettably we are unable to detect any constant differences in shell morphology by which they may be distinguished. Fortunately, their radulae are very different (Figures 6G-J), P. gamma n. sp. having more numerous cusps (16 vs. 11), so by definition they are not sibling species. The two species appear to be allopatric. all specimens sequenced as P. beta occurring off the Solomon Islands, while specimens sequenced as P. gamma occurred further south off Vanuatu. Compared with P. obtusa (Thiele, 1925) at the same stage of growth, like P. beta n. sp., P. gamma n. sp. differs in having a more elongate P1 and stronger, more widely spaced commarginal ridges and more crowded radial costae.

Vanuatu and New Caledonia, living at 532-880 m