52 



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



Olgambonites insolita sp. nov. PI. 8. figs 1 1, 14-20 



Etymology. After insolitus, Latin -extraordinary. 



Holotype. BC 56663, a dorsal valve (PL 8, figs 19, 20) from the 

 Anderken Formation, Sample 8255, Anderkenyn-Akchoku section. 



Material. 3 ventral (BC 56534-36, BC 56664) and 2 dorsal 

 valves (BC 56538, BC 56663) from the type locality. 



Description. Shell convexoconcave, semielliptical in outline, 

 about half as long as wide, with maximum width at the hinge line, 

 and acute cardinal extremities. Ventral valve gently concave in 

 lateral profile with apsacline interarea and narrow convex 

 pseudodeltidium. Dorsal valve gently convex with anacline 

 pseudointerarea. Radial ornament inequally parvicostellate with 7- 

 9 primary accentuated ribs and two generations of accentuated 

 costellae. Very fine parvicostellae, about 10-1 1 per mm. Concentric 

 ornament of fine evenly spaced rugellae and very fine crowded 

 growth lamellae in the anterior half of the shell. 



Ventral interior with small teeth and highly raised, rounded, 

 subrectangular muscle field bordered anteriorly by the steep rim. 

 Dorsal interior with simple undercut cardinal process joined to the 

 socket ridges and small alveolus. Bema entire, highly raised anteriorly 

 and bordered by a steep rim. Short median ridge originating anterior 

 to bema and joined to the subperipheral rim. 



Discussion. The new species is known only from the type locality. 



Genus ZHILGYZAMBONITES gen. nov. 

 Etymology. After Zhilgyz well, Betpak-Dala Desert. 



Type species. Zhilgyzambonites e.xtenuata sp. nov. from the 

 Anderken Formation, Chu-Ili Range. 



Diagnosis. Shell concavoconvex, with rectimarginate posterior 

 commissure, ventral interarea apsacline with delthyrium completely 

 covered by pseudodeltidium; dorsal interarea anacline with com- 

 plete chilidium; radial ornament finely and unequally parvicostellate; 

 ventral valve with small teeth lacking dental plates; ventral muscle 

 field small, highly raised anteriorly; dorsal interior with undercut 

 cardinal process joined to socket ridges, deep alveolus and strongly 

 elevated, entire bema; median ridge fine, originating anteriorly to 

 bema and joined anteriorly to the subperipheral rim. 



DISCUSSION. Zhilgyzambonites is somewhat similar to Aulie 

 (Nikitin & Popov in Klenina et al. 1984). which is within the Hes- 

 peromenidae, in the characteristic pseudodeltidium and chilidium. 

 the ventral valve, which lacks dental plates and has a small muscle 

 field which is strongly raised anteriorly, and the dorsal interior with 

 the deep alveolus and an undercut cardinal process: but it differs in 

 having an elevated bema and a short median ridge between the 

 anterior margin of the bema and the subperipheral rim. However, the 

 presence of a bema in Zhilgyzambonites places it within the 



Table 17 Measurements of ventral valves of Zhilgyzambonites extenuata 

 sp. nov., Samples 2531, 8251. 8255, from Anderkenyn-Akchoku section. 



Lv 



W 



LvAV 



N 

 X 



s 



MIN 

 MAX 



7 



3.7 



0.78 



2.2 



4.4 



7 



7 



5.2 



70.9% 



0.97 



5.4 



3.2 



61.1% 



6.2 



78.6% 



Sowerbyellidae on the criteria established by Cocks & Rong (1989). 

 The dorsal interior of the new genus somewhat resembles 

 Diambonioidea (Zeng 1987), but the Kazakh genus differs from the 

 latter in having a strongly raised ventral muscle field, a ventral 

 median ridge anterior to the muscle field, and an undercut cardinal 

 process; the simple, not undercut, cardinal process of Diambonioidea 

 places it within the Grorudiidae (Cocks & Rong 2000). 



Zhilgyzambonites extenuata sp. nov. 



PI. 8, figs 3, 4, 6-10. 12, 13 



Etymology. After extenuatus, Latin - little, weak. 



Holotype. BC 12915, a dorsal valve (PI. 8, figs 12, 13) from the 

 Anderken Formation, Sample 8255, Anderkenyn-Akchoku section. 



Material. 2 pairs of conjoined valves, 10 ventral and 10 dorsal 

 valves from Samples 2531 (BC 57490, 92), 8255 (BC 12915-21, 

 57493, 94) and possibly 82 1 5 ( BC 57089, 9 1 , 5749 1 ), Anderkenyn- 

 Akchoku section. 



Description. Shell concavoconvex, semielliptical in outline, on 

 average 70% as long as wide with maximum width at the hinge 

 line. Cardinal extremities nearly right angled. Ventral valve mod- 

 erately and evenly convex in lateral profile with apsacline interarea 

 and narrow convex pseudodeltidium completely covering the 

 delthyrium. Dorsal valve moderately concave with linear, anacline 

 interarea and convex chilidium. Radial ornament unequally 

 parvicostellate with 7-9 accentuated primary ribs and two genera- 

 tions of accentuated costellae. Parvicostellae very fine, about 

 10-11 per mm. Concentric ornament of fine evenly spaced 

 rugellae and very fine crowded growth lamellae in the anterior 

 half of the shell. 



Ventral valve interior with small teeth and highly raised, rounded, 

 subrectangular muscle field about 20% valve length and bordered 

 anteriorly by a steep rim. Dorsal interior with simple undercut 

 cardinal process joined to narrow, strongly curved socket ridges; 

 small alveolus. Dorsal adductor scars with anterior pair slightly 

 larger than posterior, strongly impressed, divided by a pair of 

 transmuscle septa. Bema entire, strongly raised anteriorly and bor- 

 dered by a steep rim. Short median ridge originating anteriorly to 

 bema and joined to the subperipheral rim. 



Discussion. As far as is yet known, this is the only species within 

 the new genus. The material from Sample 8215 does not include 

 valve internals and is thus only doubtfully referred to the species. 



Table 18 Measurements of dorsal valves of Zhilgyzambonites extenuata sp. nov.. Samples 253 1 . 825 1 . 8255. from Anderkenyn-Akchoku section. 





Ld 



W 



Ml 



Mw 



LP1 



LPw 



BB1 



BBw 



LdAV 



Ml/Ld 



Ml/Mw 



PLl/Ld 



LPwAV 



BBwAV 



N 



8 



8 



3 



3 



3 



3 



3 



3 



8 



3 



3 



3 



3 



3 



X 



3.8 



6.1 



1.5 



2.6 



2.9 



4.8 



0.3 



2.3 



62.5% 



35.1% 



58.6% 



67.4% 



73.9% 



35.7% 



s 



0.53 



0.67 



0.17 



0.40 



0.21 



0.20 



0.06 



0.72 



6.0 



2.4 



10.3 



7.2 



0.3 



12.6 



MIN 



3 



5.2 



1.4 



2.2 



2.7 



4.6 



0.3 



1.7 



54.0% 



33.3% 



46.7% 



62.2% 



73.5% 



26.2% 



MAX 



4.5 



6.8 



1.7 



3 



3.1 



5 



0.4 



3.1 



71.7% 



37.8% 



65.4% 



75.6% 



74.2% 



50.0%Order 



