58 



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



Table 19 Measurements of complete shells of Placotriplesia spissa sp. nov.. Sample 2538. Kujandysai section, 822 1 from Anderkenyn-Akchoku section 

 and Sample F- 104 la from Burultas valley. 





L 



W 



T 



Sw 



St 



LAV 



T/L 



SwAV 



St/Sw 



N 



7 



7 



7 



7 



3 



7 



7 



7 



3 



X 



13.9 



19.0 



11.5 



10.4 



7.7 



73.9% 



81.6% 



75.5% 



73.2% 



s 



2.61 



4.27 



3.13 



1.43 



3.04 



7.9 



12.7 



6.1 



22.3 



MIN 



10.5 



13.6 



6.8 



8.2 



3.3 



60.4% 



61.7% 



67.9% 



37.5% 



MAX 



16.3 



24.5 



14.7 



11.5 



7.6 



83.9% 



90.2% 



83.0% 



69.1% 



Placotriplesia, which is otherwise known only from the Silurian. It 

 differs from P. praecipta, P. juvenis Ulrich & Cooper (1936a), P. 

 waldronensis (Miller & Dyer), and P. rostellata (Ulrich & Cooper 

 1936a) in having a strongly dorsibiconvex shell, high dorsal median 

 fold and deep ventral median sulcus originating at some distance 

 from the umbo. It also differs from P. praecipta, as redefined by 

 Amsden (1968). in having a strongly curved cardinal process with 

 posteriorly-directed prongs. 



Order PROTORTHIDA Schuchert & Cooper, 1931 



Superfamily PROTORTHOIDEA Schuchert & Cooper, 1931 



Family SKENIDHDAE Kozlowski, 1929 



Genus SKENIDIOIDES Schuchert & Cooper, 1931 



TYPE SPECIES. Skenidioides billingsi Schuchert & Cooper, 1931, 

 from the Caradoc of Ontario, Canada. 



Skenidioides sp. 



PI. 10, figs 20-22 



Material. One pair of conjoined valves (L=2.7. W=5.4, T=1.7), 

 one ventral and one dorsal valves from Samples 8223b and 8226, 

 Anderkenyn-Akchoku section; Sample 8214 (BC 57518), west side 

 of Ashchisu River. 



Discussion. The shells from the Anderken Formation closely 

 resemble Skenidioides anthonensis (Sardeson). as redescribed and 

 illustrated by Cooper (1956: 491 ), in the general shape of the shell, 

 narrow median sulcus in the dorsal valve, carinate ventral valve and 

 characters of radial ornament, but differ somewhat in having a more 

 flattened dorsal valve. Although the exterior is characteristic of 

 Skenidioides. the absence of known interiors in the specimens from 

 the Anderken Formation precludes specific identification. 



Order ORTHIDA Schuchert & Cooper, 1932 



Suborder ORTHIDINA Schuchert & Cooper, 1932 



Superfamily ORTHOIDEA Woodward, 1852 



Family HESPERORTHIDAE Schuchert & Cooper, 1931 



Genus DOLERORTHIS Schuchert & Cooper, 1931 



Type species. Orthis interplicata Foerste, from the Niagara Group 

 (Silurian) of Indiana, U.S.A. 



Dolerorthis expressa Popov, 1980 



PI. 1, fig. 29, PI. 11, figs 1,2 



1980 Dolerorthis expressa Popov: 144, pi. 1, figs 5-7. 



Holotype. CNIGR 11/11 523, ventral internal and external moulds 

 (L=18.4, W=24.7), from the Anderken Formation, Sample 1018. 7 

 km southwest of Karpkuduk well, Kotnak Mountains. 



MATERIAL. One pair of conjoined valves, 1 2 ventral and 1 1 dorsal 

 valves, internal and external moulds, from Sample 8 1 37 (BC 57526), 



Anderkenyn-Akchoku; Sample 817, about 4 km south-west of Alakul 

 Lake; Sample 1018 (BC 57368),7 km southwest of Karpkuduk well, 

 Kotnak Mountains, south Betpak-Dala. 



Description. Shell subequally biconvex, transverse, subrect- 

 angular in outline, about 15% as long as wide with maximum width 

 anterior to hinge line. Cardinal extremities slightly rounded; anterior 

 commissure weakly unisulcate; ventral valve gently convex in lat- 

 eral profile with maximum thickness at about one-third from anterior 

 margin; ventral interarea apsacline, slightly curved in cross-section 

 with open triangular delthyrium. Dorsal valve moderately convex 

 with shallow sulcus originating at the umbo. Interarea low, planar, 

 anacline. Radial ornament variably multicostellate with costellae of 

 two to three generations. 4-6 ribs per 3 mm along the posterior 

 margin of adult specimens. Concentric ornament of fine, ridge-like, 

 evenly spaced fila, 3^4 per mm. 



Ventral interior with strong teeth supported by diverging dental 

 plates continuing anteriorly as elevated muscle bounding ridges 

 enclosing an elongate, subrhomboidal muscle field about two-fifths 

 as long as the valve. Adductor scars narrow, strip-like, completely 

 separating large, deeply impressed diductor scars of about equal 

 length. Mantle canals saccate with subparallel to slightly converging 

 vascula media. Dorsal interior with simple, ridge-like cardinal proc- 

 ess on the high notothyrial platform slightly inclined posteriorly. 

 Brachiophores high, triangular with slightly diverging bases. Weakly 

 impressed dorsal adductor scars divided posteriorly by a very short 

 median ridge. 



Discussion. This species is somewhat similar to Dolerorthis 

 tenuicostata Williams (in Whittington & Williams 1955: 406) from 

 the Lower Caradoc of Wales, but differs in having a more trans- 

 verse shell outline, lateral profile of the ventral valve with 

 maximum thickness anterior to the mid-length in full grown speci- 

 mens and a weak dorsal sulcus continuing towards the anterior 

 margin. It differs from Dolerorthis aff. hubeiensis Zeng. which 

 occurs in the Dulankara Regional Stage of north Betpak-Dala, 

 Kazakhstan (Nikitin et al. 1996), in having a subequally biconvex 

 transverse shell, finer radial ornament and more widely spaced 

 concentric fila. 



Dolerorthis pristina sp. nov. PI. 10, figs 23-29, 31-36 



ETYMOLOGY. After pristinus, Latin - former. 



Holotype. BC 57520, PI. 10, figs 26-29, 31, conjoined valves 

 (L=9.7. W= 1 2.6, T=4.2) from the Anderken Formation, Sample 620, 

 Anderkenyn-Akchoku section. 



Material. 8 pairs of conjoined valves, 15 ventral and 18 dorsal 

 valves from Samples 100 (=K-98/1970) (BC 57110-7), 620 (BC 

 57520), 626 (BC 57130-32, 57519), 8223a (BC 57158, 59), 8223b, 

 Anderkenyn-Akchoku; Sample 8214 (BC 57150-54, 57523), 

 Ashchsu River; Samples 628 (BC 57133-5), 2538 (BC 56768, 

 57141^15, 57521, 22), 8217 (BC 57156-7), Kujandysai near 

 Akchoku Mountain; Sample 948 (BC 57136-40), Tesik River. 



