120 



L.R.M. COCKS AND ZHAN REN-BIN 



similar to each other, as Reed himself recognised, and can probably 

 be reassigned to Porambonites (see below). The ventral exterior 

 identified by Reed (1906: 65, pi. 5, figs 13, 13a.) as Clitambonitescf. 

 pyron (Eichwald), also from the Naungkangyi Group at Sedaw 

 (about 15 km northwest of Kyaukme), Northern Shan States, cannot 

 be revised here owing to lack of material; it may or may not be a 

 clitambonitoid. 



Superfamily PLECTAMBONITOIDEA Jones, 1928 



Family LEPTELLINIDAE Ulrich & Cooper, 1936 



Subfamily LEPTELLININAE Ulrich & Cooper, 1936 



Genus LEPTELLINA (LEPTELL1NA) Ulrich & Cooper, 1936 



Leptellina (Leptellina) minor sp. nov. 



PI. 2, figs 13-14; PI. 3, figs 1-5 



1936 Leptelloidea (Leangella?) cf. derfelensis Jones; Reed: 43, 



pi. 4, figs 24-25. 

 1936 Leptelloidea (Leangella) cf. sholeshookensis Jones; Reed: 



43, pi. 4, fig. 28. 



HOLOTYPE. BC 52418 (PI. 3, fig. 2), from the equivalent of the 

 Upper Naungkangyi Group at Linwe ( AM78 ), Southern Shan States, 

 longitude 96°33'E, latitude 21°14'N. 



Material and localities. 107 specimens: 10 dorsal valves, 23 

 dorsal internal and 14 external moulds, four ventral valves, 33 

 ventral internal and 13 external moulds from the equivalents of the 

 Upper Naungkangyi Group at Linwe (Localities AM77, AM78); 

 moulds of two ventral interiors and one ventral exterior and counter- 

 part interior from the Bryozoan Sandstone Formation (equivalent to 

 the Upper Naungkangyi Group) in the Neyaungga-Ye-ngan area 

 (Localities BA479, BA490); both in the Southern Shan States. Four 

 ventral internal and two external moulds from the Li-lu Formation 

 (equivalent to the Upper Naungkangyi Group) at Ta-Pangtawng 

 (about 10 km east of Longtawkno, Locality YA454.1), Northern 

 Shan States. 



Description. Exterior. Small semicircular shell 3.0-6. 3mm long 

 and 3.4-8.4mm wide, with length/width ratio 0.66-0.88. Lateral 

 profile strongly concavo-convex, dorsal valve often dorsally genicu- 

 late at about 60% of length, strongly convex ventral valve particularly 

 medially. Cardinal extremities acute to nearly rectangular; maxi- 

 mum width along the hinge line. Large, flat and apsacline ventral 

 interarea; posterior half of the delthyrium covered by well-devel- 

 oped arched pseudodeltidium (PL 3, fig. 1 a). Smaller anacline dorsal 

 interarea; open notothyrium mostly occupied by cardinal process 

 lobes. Parvicostellate ornamentat with 4-5 finer costellae between 

 each pair of coarser ones. No growth lines observed. 



Ventral interior. Small teeth with variably developed dental plates 

 weak or absent and extending first subparallel and then medially to 



enclose the muscle field. Delthyrial cavity very deep. Rectangular or 

 transversely elliptical muscle field elevated from the shell floor, 17- 

 27% of the length and 21-34% of the width; triangular median 

 adductor scars often more elevated than the lateral diductor scars. 

 Vascular markings lemniscate; a pair of strong vascular media 

 originate from the antero-lateral ends of muscle field, extending 

 forward subparallel and branching at about two-thirds of the length; 

 a pair of very weak vascular spondylaria originate laterally from the 

 muscle field, extending to the shell lateral margin with few branches. 

 Dorsal interior. Small transverse cardinalia 16-21% of the length 

 and 28-31% of the width; strong median cardinal process lobe 

 projecting posteriorly and ventrally and continuous with the low and 

 wide myophragm anteriorly; small lateral pair of lobes variably 

 developed, sometimes absent; socket ridges connected with cardinal 

 process medially and extending laterally subparallel to the hinge 

 line, with two strong ventrally projecting lateral ends; sockets 

 transverse and elliptical. Poorly-impressed circular muscle field just 

 in front of the cardinalia, antero-medial pair of adductor scars 

 slightly larger than the lateral pair. Thin median septum starting 

 from the posterior end of the inner adductor scars, becoming higher 

 anteriorly and reaching its acme at the junction with the platform at 

 the strongest valve convexity. Weakly-elevated platform composed 

 of a series of continual or continuous tubercles, not connected with 

 hinge line posteriorly. Lemniscate vascular markings with a pair of 

 vascular media originating from the anterior ends of inner adductor 

 scars. 





L 



W 



LAV 



L, 



L,/L 



W, 



W,AV 



BB37590, dorsal valve 



5.2 



7.8 



0.67 



1.1 



0.21 



2.2 



0.28 



BB37625, dorsal valve 



5.6 



7.5 



0.75 



0.9 



0.16 



2.1 



0.28 



BB37635. ventral valve 



4.6 



6.1 



0.75 



1.2 



0.26 



2.1 



0.34 



BB37647, dorsal valve 



4.4 



5.8 



0.76 



0.8 



0.18 



1.8 



0.31 



BB37652, ventral valve 



4.0 



6.1 



0.66 



0.9 



0.23 



1.5 



0.25 



BB37742, ventral valve 



3.0 



3.4 



0.88 



0.5 



0.17 



0.8 



0.24 



BB37755, ventral valve 



6.3 



8.4 



0.75 



1.7 



0.27 



1.8 



0.21 



Discussion. This species is the most abundant component of our 

 fauna. Reed (1936: 43) identified a probably identical ventral inte- 

 rior as Leptelloidea (Leangella) cf. sholeshookensis Jones from 

 rocks corresponding to the Naungkangyi Group at Taunggyi in the 

 Southern Shan States. The true Leptelloidea sholeshookensis (Jones 

 1928: 488, pi. 15, fig. 19) has an undercut cardinal process and well- 

 developed bema and platform, and has been reassigned to Leangella 

 (Leangella) by Cocks & Rong (1989: 116). Reed also identified 

 other specimens as Leptelloidea (Leangella ?) cf. derfelensis Jones, 

 but again that Welsh species has been reassigned to Leangella 

 (Leptestiina) by Cocks & Rong (1989: 116). Reed illustrated no 

 dorsal valves. Thus, with our more complete material than Reed, we 

 can erect the new species minor, which we assign to Leptellina 

 (Leptellina). It differs from the type species L. tennesseensis (named 

 by Ulrich & Cooper in 1936: 626, but illustrated by Ulrich & Cooper 

 in 1938: 192, pi. 39, figs 1-2, 4-5), from the Lenoir Formation 



PLATE 3 



Figs 1-5 Leptellina (Leptellina) minor sp. nov. 1-4, Linwe, Locality AM78. la, lb, BB 37623, latex cast and internal mould of ventral interior showing 

 poorly developed pseudodeltidium and small teeth, x 5. 2a, 2b, BC 52418, Holotype, latex cast and internal mould of dorsal interior with another dorsal 

 interior at the top left (BB 37590), x 5. 3, BB37659, dorsal internal mould, x 4. 4, BB 37629, latex cast of dorsal exterior, showing dorsal interarea and 

 chilidium, x 4. 5, BC 52417, Ta-Pangtawng, Locality YA454. 1 , ventral internal mould, x 6. 



Figs 6-9 Bekkerella subcrateroides (Reed). 6-8. Kunkaw. 6, BB 37768, Locality YA50.1, latex cast of dorsal exterior, x 4. 7a-d, BB 37759, Locality 

 YA256, latex cast of exterior, internal mould, latex cast of interior and posterior view of internal mould of ventral valve together with a dorsal external 

 mould (BC 52410). x 4. 8a, 8b, BB 37774, Locality YA3 15.1, dorsal internal mould, and latex cast, x 3. 9a, 9b, BB 37750, Namyun, latex cast and 

 internal mould of dorsal interior, x 2. 



Fig. 10 lshimia subdeltoidea (Reed). B 29672, Tawmawgon, latex cast and internal mould of dorsal interior, x 2. 



