74 



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



convex in lateral profile with maximum thickness slightly posterior 

 to mid-valve. Ventral sulcus originating near the umbo, broad and 

 shallow with low trapezoidal tongue slightly inclined anteriorly. 

 Lateral slopes steep, slightly convex in cross-section. Dorsal valve 

 with moderately convex lateral profile strongly curved posteriorly. 

 Median fold shallow, slightly rounded in transverse section, origi- 

 nating near the beak, flanked by two strong plications. Radial 

 ornament with angular ribs originating near mid-length, 1-3 ribs in 

 the sulcus and 2-4 in the median fold. Pair of ribs occasionally on the 

 lateral slopes of both valves. Ventral interior with deep, narrow 

 spondylium about one-sixth valve length, supported posteriorly by 

 high median septum, free anteriorly. Dorsal interior with separated 

 outer plates, slightly diverging distally, short subtriangular brachial 

 plates and well-defined brachial processes. 



Measurements, conjoined valves, L=7.9, W=8.6,T=5.9, Sw=6.9; 

 conjoined valves, L=9.8, W= 11.2, T=6.7, Sw=8.2; conjoined valves, 

 L=7.8, W=8.2, T=6.2, Sw=6.8; conjoined valves, L=6.8, W=7.2, 

 T=6.2, Sw=5.4; conjoined valves, L=5.7, W=5.8, T=3.4, Sw=4.6. 



Discussion. Schizostrophina margarita, the type species of the 

 genus, came from the same unit and locality, the Jinhe Formation 

 (Lower Caradoc) of north-west China, as S. shaanxiensis and differs 

 from the latter in having a single poorly defined rib in the ventral 

 sulcus and smooth lateral sides of the valve. The Kazakh shells 

 demonstrate a strong variability in the number and characters of 

 radial ornament with growth. The small shells (about 5-6 mm long) 

 usually lack ribs on the lateral sides of the shell, and the radial 

 ornament in the ventral sulcus and dorsal fold is poorly developed or 

 absent (PI. 14, figs 22-24), whereas mature specimens possess a 

 radial ornament closely comparable to one of the specimens referred 

 by Fu (1982) to S. shaanxiensis. This suggests that all the shells 

 described by Fu represent a single species, which should be termed 

 S. margarita. Since there appear to be no constant morphological 

 differences between the Kazakh and Chinese specimens of 

 Schizostrophina. they are regarded here as conspecific. 



2538, Kujandysai Section; one dorsal internal mould (BC 57366) 

 from Sample 1018, 7 km southwest of Karpkuduk well, Kotnak 

 Mountains. 



Description. Shell smooth, subequally biconvex, transverse to 

 suboval in outline with uniplicate posterior commissure. Ventral 

 valve lateral profile moderately convex with maximum thickness 

 about one-third valve length. Ventral beak swollen and slightly 

 curved. Sulcus originating near mid-length, strongly deepening 

 towards the anterior margin, with low median rib. Tongue 

 semielliptical. Dorsal valve moderately convex in lateral profile with 

 maximum thickness at about two-thirds valve length. Median fold 

 originating somewhat posteriorly to mid-valve, clearly separated 

 from the slightly convex lateral sides of the valve. Ventral interior 

 unknown, except for median septum, possibly supporting 

 spondylium. Dorsal interior with separated, long, subparallel outer 

 plates. 



Measurements, conjoined valves, L=13.4, T=8.2. 



Discussion. The Kazakh specimens closely resemble Didymel- 

 asma transversa Fu, 1982, from the Caradoc Jinhe Formation of 

 northwest China, in the external features of their smooth shells, 

 including the shape of the dorsal median fold and ventral sulcus, but 

 our material is insufficient to make a precise attribution to this 

 species. Fu (1982, text-fig. 23) also demonstrated the presence of 

 separated, subparallel outer plates in the Chinese shells, which 

 supports their assignation to Didymelasma. 



Order RHYNCHONELLIDA Kuhn, 1949 



Superfamily RHYNCHOTREMATOIDEA Schuchert, 1913 



Family RHYNCHOTREMATIDAE Schuchert, 1913 



Genus RHYNCHOTREMA Hall, 1860 



TYPE SPECIES. Atrypa inerebescens Hall, 1847, from the Trenton 

 Formation (Caradoc), New York, U.S.A. 



Genus DIDYMELASMA Cooper, 1956 



TYPE SPECIES. Didymelasma longicrurum Cooper, 1956 from the 

 Lebanon Formation (Caradoc), Tennessee, U.S.A. 



Didymelasma cf. transversa Fu, 1982 



PI. 13, figs 75-77, PI. 14, fig. 1 



Rhynchotrema akchokense sp. nov. PI. 14, figs 28^42 



Etymology. After Akchoku Mountain on the east side of 

 Kopalysai. 



Holotype. BC 57579, PI. 14, figs 28-32, conjoined valves, from 

 the Anderken Formation, Sample 626, Anderkenyn-Akchoku sec- 

 tion. 



Material. One pair of conjoined valves, BC 57573, from Sample Material. 11 conjoined valves, five ventral and six dorsal valves 



PLATE 14 



Fig. 1 Didymelasma cf. transversa Fu, Sample 1018. area 7 km SW of Karpkuduk well, Kotnak Mountains, BC 57366, dorsal internal mould, x 2. 

 Figs 2-27 Schizostrophina margarita Fu, Sample 2538, Kujandysai section. 2—6, BC 57574, conjoined valves, dorsal, ventral, posterior, lateral and 



anterior views, x 3. 7-9, 11, 14, BC 57575, conjoined valves, anterior, dorsal, ventral, lateral and posterior views, x 3. 10, 12, 15-17. BC 57576, 



conjoined valves, anterior, posterior, dorsal, lateral and ventral views, x 3. 13, CNIGR 5/12361, dorsal exterior, x 3. 18-22, BC 57577, conjoined valves, 



lateral , dorsal, ventral, anterior and posterior views, x 3. 23-27, BC 57578. conjoined valves, ventral , dorsal, lateral, posterior and anterior views, x 3. 

 Figs 28-42 Rhynchotrema akchokense sp. nov. 28-32, 38-41, Sample 626, Anderkenyn-Akchoku section; 28-32, BC 57579, conjoined valves, holotype. 



ventral, lateral, dorsal, posterior and anterior views, x 2; 38-41, BC 57582, conjoined valves, dorsal, ventral, anterior and lateral views, x 3. 33-36, 



Sample 2538, Kujandysai section, BC 57580, conjoined valves, anterior, dorsal, ventral and lateral views, x 2. 37, 42. Sample 843. Anderkenyn-Akchoku 



section; 37, BC 57581, ventral internal mould, x 3: 42, BC 57583, dorsal internal mould, x 5. 

 Figs 43-46 Pectenospira pectenata Popov, Nikitin & Sokiran, Sample 948, Tesik River, BC 57320, dorsal, anterior, ventral and lateral views of conjoined 



valves, x 5.5. 

 Figs 47-57 Nikolaispira guttula sp.nov. 47-53, Sample 2538. Kujandysai section; 47-49, BC 57584, conjoined valves, anterior, dorsal and ventral views, 



x 3; 50-53. BC 57585. conjoined valves, holotype. ventral, anterior, lateral and dorsal views, x 3. 54-57, Sample 8221, Anderkenyn-Akchoku section, 



BC56770, conjoined valves, anterior, ventral, dorsal and lateral views, x 4. 

 Figs 58-61 Kellerella misiusi Popov, Nikitin & Sokiran, Sample 8214, Anderkenyn-Akchoku section. BC 56773, conjoined valves, anterior, ventral, 



lateral and dorsal views, x 3. 



