66 



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



Fig. 15 Photographs of transverse serial sections, x 12. Distance in mm is measured from the posterior tip of ventral beak. Dorsal valve uppermost. 1-2, 

 Placotriplesia spissa sp.nov., BC 57605, Sample 628, Kujandysai section; 1, 1.5 mm; 2, 1.8 mm. 3, 4, 9-14, Ilistrophina tesikensis gen. et sp. nov., 

 Sample 948, Tesik River, 3,4, BC 57606, 0.3 and 0.4 mm; 9-14, BC 57607, 0.4, 0.6, 0.8, 1.0 and 1.4 mm; 5-7, Parastrophina iliana sp. nov., BC 56560, 

 Sample 948, Tesik River; 0.6, 0.8 and 1.2 mm; 8, Parastrophina plena Sapelnikov & Rukavishnikova, BC 57564. Sample 948. Tesik River, 1.0 mm. 



about 66% valve width. Dorsal valve strongly convex with maxi- 

 mum thickness at mid-valve or slightly anteriorly. Umbonal area 

 strongly swollen. Broad median fold originating near mid-valve. 

 Radial ornament of coarse rounded-angular ribs originating anterior 

 to mid- valve in mature specimens with 1-3 ribs in the sulcus, 2-4- on 

 the median fold and up to 6 on the flanks of both valves. 



Ventral interior with small teeth and narrow spondylium on a low 

 median septum extending anteriorly to mid-valve. Dorsal interior 

 with narrow cruralium on the low median septum. Alate plates small, 

 projecting anteriorly as short brachial processes. Inner plates narrow, 

 gently curved. Outer plates nearly straight in cross-section, converg- 

 ing towards a thin median septum. 



Measurements. (47 1/1 2375) conjoined valves, L= 14.0, W=l 6.9, 



T=6.3, Sw=8.9; (474/12375) conjoined valves, L=20.4, W=21.0, 

 T=11.2, Sw=8.7; (475/12375) conjoined valves, L=22.9, W=24.5, 

 T=10.2, Sw=12.8; (476/12375) conjoined valves, L=16.5, W=19.8, 

 T=6.5, Sw=11.2; (479/12375) conjoined valves, L=10.5, T=6.7, 

 Sw=6.7. 



Discussion. The Kazakh shells are comparable to Parastrophina 

 haemiplicata (Hall), and in particular with specimens described and 

 illustrated by Cooper (1956: 606, pi. 106, figs 33^14; pi. 117, figs 

 19-27) from the lower Martinsburg Formation of Virginia, in outline 

 and profile of both valves, as well as in the characters of the radial 

 ornament, dorsal median fold and ventral sulcus, but they differ in 

 having more conspicuous ribbing on the flanks of mature shells, 

 which may possess up to 7 ribs; however, the number of ribs is 



Table 23 Measurements of complete shells of Parastrophina iliana sp. nov.. Sample 948, Tesik river. 





L 



W 



T 



Ld 



Sw 



St 



LAV 



T/L 



Ld/W 



Sw/W 



N 



11 



10 



10 



11 



10 



10 



10 



10 



10 



9 



X 



8.2 



9.1 



5.8 



8.1 



6.1 



3.6 



89.2% 



74.0% 



87.7% 



65.6% 



S 



2.05 



2.87 



1.16 



2.10 



1.65 



1.52 



6.3 



10.4 



5.2 



6.5 



MIN 



5.5 



5.6 



4.2 



5.3 



3.2 



1.6 



80% 



62% 



79% 



57% 



MAX 



11.9 



14.8 



7.4 



11.7 



8.5 



6.6 



98% 



95% 



95% 



78% 



