Amusium Röding, 1798

Dijkstra, Henk H. & Beu, Alan G., 2018, Living Scallops of Australia and Adjacent Waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae), Records of the Australian Museum (Rec. Aust. Mus.) 70 (2), pp. 113-330 : 204

Amusium Röding, 1798

Amusium Röding, 1798: 165 . Type species (by subsequent designation, Herrmannsen, 1846): Ostrea pleuronectes Linnaeus, 1758 ; Recent , Indonesia.

Pleuronectia Swainson, 1840: 388 . Type species (by monotypy): Pleuronectia laevigata Swainson, 1840 (= Ostrea pleuronectes Linnaeus, 1758 ); Recent, Indo-West Pacific.

Figure 46. Distribution of Amusium pleuronectes (Linnaeus) .

Diagnosis. Amusiini with (sub)circular shape, up to c. 100 mm high, very weakly inflated, both valves slightly convex; umbonal angle c. 120°; exterior surface of disc and auricles smooth, interior with c. 22–34 narrow, prominent radial riblets clearly arranged in pairs; auricles equal in length, intermediate hinge teeth absent, byssal notch and ctenolium weak in early ontogeny, absent later.

Distribution. Late Oligocene?–Recent. Tropical and subtropical Indo-West Pacific, living in the littoral zone. Amusium subcostatum Beu & Darragh (2001: 174, figs 63B, D, E, 64D, E) (Australian Junjukian and Longfordian Stages, late Oligocene and early Miocene) is doubtfully referred to Amusium (sensu stricto); it is a thick-shelled species with wide auricles and faint radial ribbing, gradational with Amussiopecten .

Discussion. Hertlein (1969: N349) placed Amusium in the Amusium group, together with Korobkovia Glibert & van der Poel, 1965 (Miocene) and Propeamussium de Gregorio, 1884 (Jurassic to Recent) and stated that these “genera may have been derived from different groups”. Propeamussium is currently included in family Propeamussiidae .

Waller (1991: 38) suggested that true Amusium evolved from Amussiopecten Sacco, 1897a during the Oligocene within the Pacific region, and placed Amusium in the Pecten group, and subsequently in Pectinini ( Waller, 1993) . Most recently, Waller (2006a) subdivided Pectinini , and transferred Amusium to the new tribe Amusiini .

Dijkstra (1990b) introduced Dentamussium as a new subgenus of Amusium . Amusium (Dentamussium) differs from Amusium sensu stricto by having a well-developed byssal notch (weak in Amusium ) and a functional ctenolium (absent at least in late ontogeny of Amusium ).