Meristorygma donakovae Poletaev 2018

Meristorygma donakovae Poletaev, 2018 Fig. 4A Meristorygma donakovae Poletaev, 2017: 74 (nom. nud.). Meristorygma donakovae Poletaev, 2018: 88, pl. 112 fig. 3. Spirifer sp. – Yanishevsky 1900: 150, pl. 1 fig. 7. Spirifer aff. gracilis – Yanishevsky 1910: 96, pl. 12 fig. 17. Brachythyris integricostus – Donakova 1983: pl. 2 figs 6– 7. Diagnosis Transversely-ovate shell with one to two pairs of ribs in the sulcus, small rounded tongue and relatively broad, scanty ribs on the lateral slopes. Material examined Holotype (Fig. 4) RUSSIA • South Urals, Magnitigorsk region, right bank of Ural River; Kizil Formation, upper Visean and Serpukhovian; Donakova 1983 Coll.; CNIGR Museum 12065/8. Paratypes RUSSIA • 2 specs; same data as for holotype; CNIGR 12065/9, 12065/10. Additional material RUSSIA • 20 specs; same data as for holotype; CNIGR 12065. Description Shell small to medium-sized; ventral valve with transversely-ovate outline, approximately equidimensional (holotype with length 26 mm, width 28 mm, thickness 18 mm), thin-walled. Hinge margin shorter than greatest width; interarea low, triangular, anacline to apsacline, smooth. Sulcus moderately developed, well delineated, with median furrow and two, rarely four primary ribs arising from parietal ribs. Anterior margin uniplicate with a small rounded tongue. Ribs on lateral slopes are few (5–7), well expressed. Dorsal valve oval in outline, almost such convex as ventral valve. Fold with three distinct ribs, of which two lateral ribs weakly bifurcate; interspaces between ribs broad. Surface of ribs smooth with concentric traces of growth. Ventral adminicula absent. Features of vascular system not established. Remarks This species differs from Meristorygma arctica by wider transversely oval outlines of the shell and a short, rounded tongue with a pair of weak additional ribs on its slopes, whereas M. arctica has a long, flat and subquadrate tongue with a pair of ribs. Meristorygma donakovae differs from the geographically and stratigraphically closest species M. gracilis by wider and less numerous ribs on the lateral slopes of both valves. The specimens examined resemble M. postgracilis in outline and rib pattern, but the latter has a narrower shell, more frequent bifurcation of the ribs in the sulcus, and a subquadrate tongue. Meristorygma parva (Rotai, 1951) differs in small size and low total number of ribs (not more than eight); M. miussensis and M. panduriformis in contrast are much larger and have very rare bifurcation of ribs in the sulcus.