UPPER ORDOVICIAN BRACHIOPODS FROM KAZAKHSTAN 



71 



sessile crurallum 



1(0-2) 2(0.3) 3(0.4) 4(05) ^Y(0. 6) 



11 (1.5) 



median septum 



Fig. 18 Transverse serial sections of Liostrophia pravula sp.nov.. Sample 2538, Kujandysai section. Distance in mm is measured from the posterior tip of 

 ventral beak. Dorsal valve uppermost. Also lateral view to show section positions and schematic reconstruction. 



T=3.4; (519/12375) conjoined valves, L=5.2, W=5.7, T=2.0; (520/ 

 12375) conjoined valves, L=7.9, W=7.9, T=3.2; (521/12375) con- 

 joined valves, L=8.7, W=8.2, T=4. 1 ; (522/12375) conjoined valves, 

 L=5.8, T=6.3, Sw=3.7.1; (523/12375) conjoined valves, L=10.6, 

 T=10.5, Sw=4.7. 



DISCUSSION. This species differs from Liostrophia glabra Cooper 

 & Kindle in having a subcircular outline, weak ventral sulcus and in 

 the absence of a dorsal median fold. The characters of the dorsal 

 interior in the type species remain inadequately known. In particular 

 it is unclear from the existing illustrations whether or not it has a 

 sessile cruralium or if it is supported anteriorly by a very short 

 septum. In external morphology, particularly in the smooth, rounded 

 shell with an uniplicate anterior commissure but without a distinct 

 dorsal median fold, Liostrophia pravula resembles Psilocamera 

 planisulcata Fu, 1982 from the the Jinhe Formation (Caradoc) of 

 north-west China. However, in the single transverse section provided 

 by Fu (1982, text-fig. 1 8A) the outer plates appear to be completely 

 separate and there are no alate plates or inner plates illustrated. 

 Liostrophia pravula differs from juvenile specimens of llistrophina 



Table 27 Measurements of ventral valves of Plectosyntrophia unicostata 

 sp. nov., samples 100 and 626 from Anderkenyn-Akchoku section. 





L 



W 



T 



St 



Sw 



LAV 



T/L 



Sw/W 



N 



6 



6 



6 



6 



6 



6 



6 



6 



X 



9.5 



10.7 



7.2 



4.0 



5.8 



89.4% 



74.1% 



156.2% 



s 



1.37 



2.10 



2.77 



1.87 



1.82 



4.6 



18.3 



28.4 



MIN 



7.8 



8.4 



4.6 



2.2 



3.5 



82.9% 



59.0% 



119.6% 



MAX 



11 



13.2 



11.2 



6.5 



7.9 



92.9% 



101.8% 



177.1% 



tesikensis not only in its larger size and sessile cruralium (which 

 hardly exceeds half the maximum length), but also in the absence of 

 a dorsal median fold. A ventral sulcus is present in llistrophina 

 tesikensis when specimens are 4-5 mm long, whereas in Liostrophia 

 pravula it is visible only in mature specimens which exceed the 

 average shell size of about 6 mm. 



Subfamily ANASTROPHIINAE Nikiforova, 1960 

 Genus PLECTOSYNTROPHIA Fu, 1982 



TYPE SPECIES. Plectosyntrophia qilianshanensis Fu, 1982. 

 the Yingou Group (Middle Ordovician) of North China. 



from 



Plectosyntrophia unicostata sp. nov. 



PI. 13, figs 59-74; Fig. 19 



Holotype. BC 57570, PI. 13, figs 67-70, conjoined valves from 

 Sample 626, Anderkenyn-Akchoku section. 



Material. Seven pairs of conjoined valves and one dorsal valve 

 from Samples 100 (=K-98/1970) (BC 57569, 71), 620 (BC57360), 

 626 (BC 57271-73, 57568, 70, 72), 8214 (BC 57361), Anderkenyn- 

 Akchoku section; Samples 2538, 8217 (BC 57362), Kujandysai 

 Section. 



DESCRIPTION. Shell subequally biconvex, transverse, subpent- 

 agonal in outline, about 75% as thick as long and about 89% as long 

 as wide with maximum width slightly anterior to mid-length. Ante- 

 rior commissure uniplicate. Ventral valve moderately convex with 

 curved beak slightly raised above a narrow triangular, apsacline 

 interarea. Ventral sulcus narrow and shallow, but with steep lateral 



