70 



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



&&OTW" 



Fig. 17 Transverse serial sections of Ilistrophina tesikensis gen. et sp. nov.. BC 57606. Sample 948. Tesik River. Distance in mm is measured from the 

 posterior tip of ventral beak (dorsal valve uppermost), also an axial diagram showing the plates. 



about 65% as high as long and 90% as long as wide with maximum 

 width at mid-length. Anterior commissure strongly uniplicate. Ven- 

 tral valve gently convex in lateral profile with maximum thickness 

 somewhat posterior to mid-length. Beak small, curved towards the 

 hinge line. Sulcus originating about 4-5 mm from the umbo, strongly 

 deepening anteriorly and terminating with a high semioval tongue 

 about 70% valve width. Dorsal valve with moderately convex lateral 

 profile. Beak slightly swollen, curved. Dorsal median fold originat- 

 ing anterior to mid-length, semioval in cross-section. 



Ventral interior with small teeth and narrow spondylium, bell- 

 shaped in cross-section, sessile posteriorly and raised anteriorly on a 

 low, thick median septum extending to mid-valve length. Dorsal 

 interior with cruralium supported by high median septum along its 

 entire length. Inner plates present. 



Measurements, conjoined valves, Lv=9.8, W=11.7. T=6.8, 

 Sw=7.8, St=4.0; conjoined valves, Lv=7.8, W=8.2, T=5.7; Sw=5.5, 

 St=4.7. 



Discussion. This species is externally comparable to Eostrophina 

 uniplicata from the Middle Ashgill Xiazhen Formation of South 

 China, but can be easily distinguished in its smaller size, which does 

 not exceed 10 mm in length in the largest specimens, and in having 

 a dorsal median fold evenly rounded in cross-section and originating 

 anteriorly to the mid-valve. 



Genus LIOSTROPHIA Cooper & Kindle. 1936 



Type SPECIES. Liostrophia glabra Cooper & Kindle. 1936, from 

 the Ashgill of Canada. 



Liostrophia pravula sp. nov. PI. 13, figs 1-29; Fig. 18 



Etymology. After pravus, Latin - bowed. 



HOLOTYPE. BC 57553, pi. 13, figs 16-20, conjoined valves, from 

 the Anderken Formation, Sample 2538, Akchoku Mountain, 

 Kuyandysai section. 



Material. 30 pairs of conjoined valves, one ventral and 7 dorsal 

 valves, from Samples 100 (=K-98/1970) (BC 57253-56), 626 (BC 

 57263-65), 8223, Anderkenyn- Akchoku section; Samples 2538 (BC 

 57266-69, 57553-55), 8256 (BC 57550-52), Kuyandysai section; 

 Sample 948 (BC 57257-62), Tesik River. 



Description. Shell slightly dorsibiconvex, transverse, subcircular 

 in outline, about 5 1 % as thick as long and 90% as long as wide, with 

 maximum width at mid-length. Anterior commissure gently 

 uniplicate. Ventral valve gently convex in lateral profile, with maxi- 

 mum thickness somewhat posterior to mid-length. Small, pointed 

 beak, slightly erect and posterior to hinge line. Broad shallow sulcus, 

 originating near the anterior margin, usually asymmetrical in cross- 

 section. Dorsal valve moderately and evenly convex in profile with 

 slightly swollen, curved beak. Dorsal median fold usually absent but 

 weakly developed anteriorly in mature specimens. Shell surface 

 smooth with one or two ribs in the fold and sulcus near the anterior 

 margin of the largest specimens. 



Ventral interior with teeth and spondylium supported anteriorly 

 by very low septum enclosed posteriorly by a secondary shell 

 thickening (Fig. 18). Dorsal interior with sessile cruralium formed 

 by high outer plates joined at the valve floor. Alate plates small, 

 appearing at some distance from the umbo. Inner plates small, 

 slightly thickened, strongly curved in cross-section with the outer 

 parts located almost within comissural plane. Brachial processes 

 thin and relatively long. 



Measurements. (518/12375) conjoined valves, L=7.6, W=7.9, 



Table 26 Measurements of ventral valves of Liostrophia pravula sp. nov.. 

 Sample 2538. Kujandysai section. 





L 



W 



T 



Sw 



St 



LAV 



T/L 



SwAV 



N 



11 



11 



11 



10 



10 



9 



9 



9 



X 



5.7 



6.3 



2.9 



4.1 



1.5 



89.7% 



50.9% 



61.8% 



s 



1.45 



1.56 



1.05 



0.75 



0.18 



7.6 



6.3 



3.7 



MIN 



3.3 



3.5 



1.3 



3.2 



1.2 



81.5% 



42.6% 



57.1% 



MAX 



8.2 



9.2 



5.1 



5.7 



1.8 



103.8% 



62.2% 



70.2% 



