32 



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



1(0.5) 2(1.3) 



16(6.5) 



10 



I 



20 mm 



j i 



Fig. 10 Transverse serial sections of conjoined valves of Teratelasmella chugaevae sp. nov. from Sample 100. 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. 



90% as thick as long. Anterior commissure strongly uniplicate. 

 Ventral valve moderately convex with maximum thickness near 

 quarter valve length. Interarea high, triangular, apsacline with 

 delthyrium completely covered by pseudodeltidium perforated by a 

 small umbonal foramen. Ventral sulcus originating at the umbonal 

 area, strongly deepening anteriorly and ending with prominent, 

 semielliptical tongue inclined at less than a right angle to the com- 

 missural plane. Flanks of the valve flattened, slightly inclined to the 

 commissural plane. Dorsal valve very strongly convex, with a low, 

 anacline interarea. Strong median fold, rounded in cross-section, 

 with steep lateral slopes originating near the beak. Very weak and 

 narrow dorsal median sulcus in the umbonal area of some specimens. 

 Radial ornament coarsely parvicostellate with 5 ribs per 3 mm along 

 the posterior margin of mature specimens. 



Interiors of both valves were studied in transverse sections (Figs 

 10, 11). Ventral valve with strong teeth supported by high dental 

 plates continuing anteriorly as muscle bounding ridges. Low median 

 ridge in the anterior half of the ventral muscle field. Dorsal valve 

 interior with bilobed cardinal process and low, curved socket ridges. 

 Median septum high, triangular, blade-like, extending anteriorly to 

 mid-valve, flanked laterally by a pair of short side septa. Adductor 

 field raised anteriorly and bordered by a high rim. 



DISCUSSION. This species is similar only to Teratelasmella plicata 



Laurie, 1991. but it can be distinguished in being larger (up to 25.6 

 mm wide), with a strongly dorsibiconvex lateral profile, a deep 

 ventral sulcus, a high dorsal median fold originating in the umbonal 

 area rather than at the mid-valve as in the type species, and coarser 

 radial ornament. 



Furcitellinae gen. et sp. indet. 



PI. 1, figs 30, 31, PI. 2, figs 1-4 



Material. Four pairs of conjoined valves, two ventral and six 

 dorsal valves from Samples 100 (BC 56859, 57382, 57384) and 626, 

 Anderkenyn-Akchoku section; Sample 628 (BC 56577, 56863, 

 57381) and 8220 (BC 56578), Kujandysai section; Sample 1041a 

 (BC 56860-62, 57383), Burultas Valley. 



Description. Shell convexoplane, transverse, subrectangular in 

 outline with maximum width near mid-length. Cardinal extremities 

 nearly right angled. Anterior commissure rectimarginate. Ventral 

 valve almost flat with acute and slightly erect beak. Ventral interarea 

 low apsacline with convex pseudodeltidium. Dorsal valve evenly 

 convex with low, orthocline interarea and well-developed chilidium. 

 Radial ornament parvicostellate. Up to 10 mm from the umbo the 

 ribs are nearly equal in size and are 7-9 per 3 mm. In larger 

 specimens 2-5 finer costellae become inserted between the larger 



