50 



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



Description. Shell concavoconvex, transverse, semielliptical in 

 outline, about 56% as long as wide, with maximum width at hinge 

 line. Cardinal extremities acute and slightly alate. Anterior commis- 

 sure broadly rounded and rectimarginate. Ventral valve moderately 

 and unevenly convex in profile with the maximum thickness at about 

 quarter valve length. Ventral interarea strongly apsacline and slightly 

 curved in cross-section, with small triangular delthyrium covered 

 apically by pseudodeltidium. Dorsal valve moderately concave, with 

 hypercline interarea; notothyrium covered laterally by chilidial plates. 

 Radial ornament unequally and finely parvicostellate with closely- 

 spaced parvicostellae varying from 9 to 1 2 per mm. Five accentuated 

 ribs originating at the umbo and four accentuated costellae inclined 

 between them in the mid-valve. Indistinct radial plications devel- 

 oped sometimes along the accentuated ribs. About 6-9 strong 

 concentric rugellae inclined below 30-40° towards the hinge line. 



Ventral interior with small teeth lacking dental plates and small 

 bilobate muscle field with flabellate diductor scars completely sur- 

 rounding small lanceolate adductor scars. Dorsal interior with 

 undercut cardinal process, small curved socket ridges, bilobed bema, 

 short median ridge and two widely divergent side septa. 



Discussion. The Anderken specimens strongly resemble 

 Anisopleurella ampla Nikitin & Popov (1996) from the slightly 

 younger Dulankara Regional Stage of north Betpak-Dala, Central 

 Kazakhstan, in size and general shell shape, but differ slightly in 

 having a finer radial ornament with 9-12 parvicostellae per mm 

 instead of 7-10 in the Dulankara specimens, and an uneven ventral 

 valve lateral profile. Anisopleurella ampla is reassigned here to 

 Sowerbyella (Sowerbyella) mainly because it has two pairs of side 

 septa including a closely-spaced more prominent pair. In the variable 

 development of strong concentric ornament it is similar to the 

 subgenus S. (Rugosowerbyella), but differs in having a median 

 septum and the two pairs of side septa, and in the small, weakly 

 impressed ventral muscle field. 



Genus ANISOPLEURELLA Cooper, 1956 



TYPE SPECIES. Anisopleurella tricostellata Cooper, 1 956, from the 

 Pratt Ferry Formation (Llandeilo) of Alabama, U.S.A. 



DISCUSSION. The relationships of Anisopleurella and other genera 

 are unresolved. The Ordovician genera within the subfamily are 

 Sowerbyella, Anisopleurella, Eochonetes, Eoplectodonta, 

 Gunningblandella and our new genera Olgambonites and 

 Zhilgyzambonites. Eochonetes and Eoplectodonta may be inter- 

 preted as Sowerbyella with hinge lines modified to form denticles 

 (which resulted in the subsequent loss of dental plates), and 



Gunningblandella and Olgambonites are resupinate modifications 

 of Sowerbyella, which was a very plastic stock. Anisopleurella has a 

 much more distinctive and erect dorsal median septum and side septa 

 than Sowerbyella, with the side septa bisecting the divided bema but 

 not reaching to the anterior edge of it. Both genera occur in rocks of 

 Llandeilo age, but which of the two was ancestral is not known. 



Anisopleurella sp. 



PI. 8, figs 1,2,5 



Material. One ventral and one dorsal internal mould from Sam- 

 ples 2531 (BC 57489), 8255 (BC 56539), Anderkenyn-Akchoku 

 section. 



Discussion. These specimens can be firmly assigned to 

 Anisopleurella because they have the characteristic parvicostellate 

 ornament with three accentuated ribs, a dorsal median ridge and a 

 pair of widely diverging prominent side septa bisecting a bilobed 

 bema. They are somewhat comparable to, and may be conspecific 

 with, Anisopleurella yichangensis Zeng, 1 987 from the early Caradoc 

 Miaopo Formation of Hubei. South China, but the absence of well- 

 preserved exteriors of the Chinese species makes detailed comparison 

 impossible. 



Genus OLGAMBONITES gen. nov. 



Etymology. After the late Olga Ivanova Nikiforova, a pioneer in 

 brachiopod studies. 



TYPE SPECIES. Olgambonites insolita sp. nov. from the Anderken 

 Formation. Chu-Ili Range. 



DIAGNOSIS. Shell convexiconcave; anterior commissure 

 rectimarginate to weakly uniplicate; ventral interarea procline to 

 slightly apsacline with apical pseudodeltidium; dorsal interarea 

 anacline with separate chilidial plates; radial ornament unequally 

 parvicostellate; ventral interior with small teeth lacking dental plates 

 and small bilobed muscle field with short adductor scars completely 

 separating larger diductor scars; ventral mantle canals lemniscate; 

 dorsal interior with simple undercut cardinal process joined to 

 narrow socket ridges; fine median ridge and bilobed bema bordered 

 by rim and crossed by up to 8 side septa. 



Discussion. Olgambonites possesses an undercut cardinal proc- 

 ess, divided bema and side septa in the dorsal valve; all characteristic 

 of the Sowerbyellidae, but it differs from most genera in the family 

 in having a convexiconcave shell. The only other resupinate genus is 

 Gunningblandella (Percival 1979b) from the Caradoc of Australia, 

 but Olgambonites differs from that genus in having a dorsal median 

 septum and numerous side septa. 



PLATE 8 



Figs 1, 2, 5 Anisopleurella sp. 1, 2, Sample 8255, Anderkenyn-Akchoku section, BC 56539, dorsal internal mould and latex cast, x 12. 5, Sample 2531. 



Anderkenyn-Akchoku section, BC 57489, ventral internal mould, x 7. 

 Figs 3, 4, 6-10, 12, 13 Zhilgyzambonites extenuata gen. et sp. nov. Anderkenyn-Akchoku section. 3, 9, 10, 12, 13, Sample 8255; 3, BC 57493, ventral 



internal mould, x 6; 9, BC 56538, latex cast of conjoined ventral and dorsal interiors, x 4; 10, BC 57494, ventral internal mould, x 6; 12, 13, BC 12915, 



holotype, latex cast and internal mould of dorsal interior, x 5.5. 4, 7, 8, Sample 253 1 ; 4, BC 57490. latex cast of ventral exterior, x 6: 7, 8, BC 57492, 



dorsal internal mould and latex cast, x 5. 6, Sample 8215, BC 57491. latex cast of dorsal exterior, x 6. 

 Figs 11, 14-20 Olgambonites insolita gen. et sp. nov. Sample 8255, Anderkenyn-Akchoku section. 11, BC 56535. dorsal external mould, x 4. 14, 15, BC 



56534. latex cast and ventral internal mould, x 4. 16, 17, BC 56664, latex cast and ventral internal mould, x 4. 18, BC 57592. ventral exterior, x 4. 19, 20, 



BC 56663. holotype, internal mould and latex cast of dorsal valve, x 4. 

 Figs 21-33 Gacella institata sp. nov. 21-25, 32, 33. Sample 100. Anderkenyn-Akchoku section; 21-25, BC 57496, conjoined valves, posterior, anterior, 



dorsal, ventral and lateral views, x 2; 32, 33, BC 57500. conjoined valves, holotype, dorsal and ventral internal moulds, x 2. 26, 27. Sample 85258, 



Kujandysai section, BC 56576, latex cast and dorsal internal mould, x 3. 28-31. Sample 626, Anderkenyn-Akchoku section, BC 57497, conjoined valves, 



anterior, ventral, lateral and dorsal views, x 2. 



