Meristorygma Carter 1974

Genus Meristorygma Carter, 1974 Type species Meristorygma arctica Carter, 1974; original designation. Diagnosis Medium-sized shell with diverse outline; fold and sulcus moderately developed. Well-defined sulcus with two, rarely four primary ribs and median furrow; uniplicate anterior commissure usually with subquadrate tongue; fold with three distinct ribs; the central with weakly furrow. Ribs on sides are few and weak. Hinge line of short, smooth ventral interarea non-denticulated. Ventral adminicula and dorsal crural plates are absent; vascula ramose, radially arranged with a pair of strong vascular media, which correspond to the pair of costae in the sulcus. Microornament is absent. Species included Arctic Canada and southeastern Alaska (Carter 1974: 674): Meristorygma arctica Carter, 1974; Pennsylvanian. Russia, Northern and Polar Ural (Kutorga 1844; Kalashnikov 1980): M. panduriformis (Kutorga, 1844); upper Pennsylvanian–Lower Permian; M. pajkchoica Kalashnikov, 1980; Moscovian. South Ural (Bashkortostan)and Middle Asia (Yanishevsky 1900; Poletaev 2018): M. gracilis (Yanishevsky, 1900) upper Serpukhovian–lower Bashkirian; (this paper) M. donakovae Poletaev, 2018; uppermost Visean–Serpukhovian, Magnitogorsk district. Ukraine, Donets Basin (Rotai 1951; Poletaev 2018): M. miussensis (Rotai, 1951) upper Bashkirian; M. postgracilis (Rotai, 1951); Moscovian. Remarks The analysis of the ventral valve of the natural mould of Meristorygma arctica (Carter & Poletaev 1998) has shown that the species has a ramose, radially arranged vascular system with a twin of strong vascular media, in contrast to Brachythyris ovalis (Phillips, 1836), which has a weak, simple, pinnate vascular system. The variability of the vascular system of related taxa may mostly be the result of evolution (Lazarev & Poletaev 1982).