Zygochlamys Ihering, 1907

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 : 273

Zygochlamys Ihering, 1907

Zygochlamys Ihering, 1907: 250 (proposed as a subgenus of Chlamys ). Type species (by original designation): Pecten geminatus G. B. Sowerby I, 1846 . San Julian Formation (late Oligocene or early Miocene), near Puerto Madryn, Argentina .

Psychrochlamys Jonkers, 2003: 43 . Type species (by original designation): Pecten patagonicus King, 1832 . (Pliocene?) Pleistocene & Recent, southern South America and the Falkland Islands .

Diagnosis. Medium to large Pedini with antimarginal microsculpture throughout ontogeny, some (Argentinian Eocene–early Miocene) species with shagreen microsculpture throughout or early in ontogeny; weak interstitial commarginal sculpture in early radial stage; scaly primary, secondary and tertiary radial macrosculpture in a characteristic fasciculate pattern, forming wide, triangular-section primary plicae on lv; dorso-ventrally elongate, with byssal notch and ctenolium prominent when young, becoming subcircular or length exceeding height when adult; byssal notch shallow and ctenolium weak in adults. Internal radial furrows present, representing internal expressions of external plicae. Hinge with moderately weak to prominent resilial and dorsal teeth.

Distribution. Eocene–Recent. Southern Hemisphere (southern Australia, New Zealand, southern Chile and southern Argentina; fossil in the subantarctic islands; Jonkers, 2003), living in the littoral to bathyal zones in colonies on soft sediment, free-lying as adults.

Discussion. Hertlein (1969: N355) treated Zygochlamys as a junior synonym of Chlamys Röding, 1798 , placed in the Chlamys group.

Waller (1991: 28) considered Zygochlamys to be an extant genus from the Southern Hemisphere, including the following species: Z. patagonica ( King, 1832) , Z. delicatula (Hutton, 1873) and the “closely related” New Zealand species Pecten dichrous Suter, 1909 [sic], and Chlamys (Mimachlamys) taiaroa Powell, 1952 , and placed it in the suprageneric Chlamys group.

Iredale (1925: 253) and Beu (1995: 18) pointed out that the type material of Talochlamys pulleineana is closely similar to Pecten (Chlamys) dichrous and that Chlamys (Mimachlamys) taiaroa is a subjective junior synonym of the former. According to Beu (1995) and Beu & Darragh (2001), Zygochlamys and Talochlamys have different origins, and C. dichroa should be placed in Talochlamys . According to Jonkers (2003) circum-subantarctic Recent species, formerly placed in Zygochlamys , belong in a distinct genus Psychrochlamys , lacking the shagreen microsculpture present on some early species of Zygochlamys . However, in our opinion middle Eocene specimens observed (AGB, with M. Griffin, Museo de La Plata, Argentina) in the Rio Turbio Formation, southern Argentina, are typical of Zygochlamys as well as very similar to Recent specimens of Z. patagonica and Z. delicatula in all characters. This group merely lost shagreen microsculpture during its evolution, and these taxa all form one clade. We follow the opinion of Beu & Darragh (2001: 118) and Dijkstra & Marshall (2008: 63) and treat Psychrochlamys as a synonym of Zygochlamys .