72 



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



Fig. 19 Photographs of transverse serial sections of Plectosyntrophia unicostata sp. nov., BC 57572 

 12. Distance in mm is measured from the posterior tip of ventral beak. Dorsal valve uppermost. 



from Sample 626, Anderkenyn-Akchoku section: x 



slopes, originating slightly posteriorly to mid- valve, and ending with 

 a shallow, narrow, trapezoidal tongue about 40% valve width. Dorsal 

 valve strongly convex in lateral profile with maximum thickness 

 near mid-length. Beak slightly swollen and strongly curved towards 

 the hinge line. A low, flattened median fold originates from the 

 umbo. Radial ornament mainly costate, with occasional bifurcating 

 ribs and with 1 primary rib in the sulcus, 2 primary ribs in the median 

 fold and 6-9 on the lateral slopes of the valves. In some specimens 

 one or two small secondary costellae originate in the median fold and 

 dorsal sulcus between the umbo and mid-length. Ventral interior 

 with strong teeth; bell-shaped spondylium in transverse section, 

 sessile posteriorly and raised anteriorly on a low median septum 

 partly covered by secondary shell (Fig. 19). Dorsal interior with 

 narrow cruralium on a high median septum extending anteriorly up 

 to 3 mm in adults. Inner plates narrow, curved; alate plates narrow, 

 bordered laterally by a pair of high subparallel muscle bounding 

 ridges. 



Measurements. (CNIGR 471/12375) conjoined valves, L=14.0, 

 W=16.9, T=6.3, Sw=8.9; (474/12375) conjoined valves, L=20.4, 

 W=21.0, T=l 1.2, 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 generic attribution of unicostata is tentative 

 because the interior of Plectostrophia qilianshanensis is inadequately 

 known. In particular, there is no record of the presence of alate plates 

 in the type species, but some illustrations provided in the original 

 description (Fu 1982, text-fig. 17) suggest their presence. The 

 characters of the ventral interior, in particular the presence of a 

 sessile spondylium slightly raised near its anterior margin on a short 

 septum, also needs confirmation, because Fu's illustrations are sche- 

 matic and it could be a preservational pattern. In the Kazakh species 

 alate plates are well defined, whereas the sessile spondylium is 

 present only in the earliest ontogenetic stages and is characterised by 

 a spondylium supported by a low median septum partly covered by 

 secondary shell. Plectosyntrophia qilianshanensis has 3 ribs in the 

 ventral sulcus and 4 in the dorsal fold, which makes it more similar 

 to Eoanastrophia kurdaica than to our species. 



P. unicostata differs from Eoanastrophia kurdaica Sapelnikov & 

 Rukavishnikova ( 1 975 ) from the Keskentas Formation (Caradoc), of 

 the Kendyktas Range, south Kazakhstan, in having a more pro- 

 nounced dorsal median fold with two primary ribs and a ventral 



