56 



L.E. POPOV, L.R.M. COCKS AND I.F. NIKITIN 



0.45 0.60 0.75 \ 



1.35 



1.50 



5 mm 



1.80 



Fig. 13 Transverse serial sections of Placotriplesia spissa sp.nov., 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=17.2, W=19.2, 

 T=8.2, Sw=9.6; (491/12375) dorsal valve, L=17.2, W=21.0, T=8.6. 

 Sw=8.2; (495/12375) dorsal valve, L=10.2,W=15.3,T=7.8,Sw=4.6. 



Discussion. This species differs from Grammoplecia triple sioides 

 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, U.S.A. 



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 575 1 7 ), 82 1 9, 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 high, planar, apsacline with flat 

 pseudodeltidium. Ventral sulcus originating from quarter to half 

 valve length, strongly deepening anteriorly, with strong geniculated 

 tongue about 75% valve width, and inclined at less than a right angle 

 towards commisural plane. Lateral sides of sulcus accentuated by 

 angular plications. Dorsal valve strongly convex, with swollen 

 incurved beak; dorsal median fold strong and rounded in cross- 

 section. Ventral interior with delicate teeth and short divergent dental 

 plates. Dorsal interior with grooved forked cardinal process with 

 strongly curved prongs posteriorly with distal parts subparallel to 

 commisural plane, separated proximally and fused with narrow 

 curved socket ridges. 



PI. 10, figs 9-18; Figs 13, 15 Discussion. This species represents the earliest record of 



PLATE 10 



Figs 1-8, 19 Triplesia aff. subcarinata Cooper. Anderkenyn- Akchoku section. 1-4, 19, Sample 825 la, BC 575 14, conjoined valves, ventral, dorsal, posterior 

 and anterior views, x 2, dorsal view of the umbonal area showing pseudodeltidium with monticulus, x 4. 5-8, Sample 100, BC 575 15. conjoined valves, x 2. 



Figs 9-18 Placotriplesia spissa sp.nov. 9-14, Sample 2538, Akchoku Mountain, Kujandysai section. 9-13, BC 575 1 7. holotype, conjoined valves, 

 ventral, dorsal, anterior, posterior and lateral views, x 2. 14, CNIGR 3/12361, conjoined valves showing ventral interarea with pseudodeltidium lacking 

 monticulum, x 5. 15-18, Sample 8214. Anderkenyn-Akchoku section, CNIGR 3/12361. conjoined valves, ventral, dorsal, anterior and lateral views, x 2. 



Figs 20-22 Skenidioides sp. Sample 8214, west side of Ashchisu River, BC 57518, conjoined valves, ventral, dorsal and posterior views, x 6. 



Figs 23-29, 31-36 Dolerorthis pristina sp. nov. 23-25, 32, 34. Sample 626. Anderkenyn-Akchoku section. 23-25, 32. BC 57519, conjoined valves, 

 ventral, lateral and dorsal views, x 2, detail of the shell surface, x 8; 34, BC 57522, ventral internal mould, x 2. 26-29, 31, Sample 620, Anderkenyn- 

 Akchoku section, BC 57520. holotype, conjoined valves, dorsal, ventral, lateral, posterior and anterior views, x 2. 33, 35, Sample 2538, Akchoku 

 Mountain, Kujandysai section; 33. BC 57521. ventral exterior, x 2; 35, BC 56768, dorsal interior, x 3. 36, Sample 8214. Anderkenyn-Akchoku section, 

 BC 57523, dorsal internal mould, x 2. 



Figs 37^12 Glyptorthis sp., Kujandysai section. 37, Sample 2538, BC 57524, ventral internal mould, x 3. 38^12, Sample 628 (=K- 107/70), west side of 

 Kujandysai, BC 57525, conjoined valves, anterior, dorsal, lateral and ventral views, x 3, dorsal view, x 6. 



Fig. 43 Plectorthis? bundtasica sp. nov. Sample 1018, area 7 km SW of Kotnak mountains, south Betpak-Dala, CNIGR 2/1 1989, latex cast of ventral 

 interior, x 2. 



