. Bulletin of the Natural Histort Museum. Geology series. . 5 mm Fig. 13 Transverse serial sections of Placotriplesia spissa , BC 57605 from Sample 628. Distance in mm is measured from the posterior tip of ventral beak. Dorsal valve uppermost. and narrow median ridge extending to anterior border of the muscle field. Measurements. (490/12375) dorsal valve, L=, W=, T=, Sw=; (491/12375) dorsal valve, L=, W=, T= Sw=; (495/12375) dorsal valve, L=,W=,T=,Sw= Discussion. This species differs from Grammoplecia triplesioides Wright &
. Bulletin of the Natural Histort Museum. Geology series. . 5 mm Fig. 13 Transverse serial sections of Placotriplesia spissa , BC 57605 from Sample 628. Distance in mm is measured from the posterior tip of ventral beak. Dorsal valve uppermost. and narrow median ridge extending to anterior border of the muscle field. Measurements. (490/12375) dorsal valve, L=, W=, T=, Sw=; (491/12375) dorsal valve, L=, W=, T= Sw=; (495/12375) dorsal valve, L=,W=,T=,Sw= Discussion. This species differs from Grammoplecia triplesioides Wright & Jaanusson, 1993, G. globosa (Nikitin & Popov, 1985), from the Andryushinka Formation (Llandeilo-Lower Caradoc) of north-central Kazakhstan, G. krotovi (Chernyschev, 1887) from the Upper Caradoc to Lower Ashgill of Novaya Zemlya, Vaigach and the Urals (Bondarev 1968) and G. sibirica (Nikiforova, 1955) from the Upper Caradoc of Siberia, in having a more transverse shell outline, a ventral sulcus and dorsal median fold with steep lateral sides and flat centrally, a strongly and evenly convex dorsal profile with maximum height at mid-length and a broader hinge line. Genus PLACOTRIPLESIA Amsden, 1968 Type SPECIES. Triplesia praecipta Ulrich & Cooper, 1936a, from the Wenlock of Arkansas, Placotriplesia spissa sp. nov. Holotype. BC 57517, PI. 10, figs 9-13, from Sample 2538, Anderken Formation, Akchoku Mountain, Kujandysai section. Material. 9 pairs of conjoined valves. 6 ventral and 12 dorsal valves from Samples 8214, 8215, 8223, Anderkenyn-Akchoku sec- tion; Samples 628,2538 (BC 57517), 8219,8256, Kujandysai section. Description. Shell smooth, dorsibiconvex profile, about 80% as thick as long and 75% as long as wide, transverse and suboval in outline, with maximum width at mid-length. Hinge-line short, less than half valve width. Anterior commissure strongly uniplicate. Ventral valve moderately convex, with an erect beak and minute apical foramen. Ventral interarea hi
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