122 



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



(Llandeilo) at Friendsville inTennessee, U. S. A., in having a smaller 

 shell, less numerous parvicostellae and less acute cardinal extremi- 

 ties. The Leptellina sinensis of Xu, Rong & Liu (1974: 152, pi. 66, 

 figs 13, 17-18) from the Shihtzupu Formation (early Caradoc) of 

 Guizhou, South China, differs from minor in having a much larger 

 shell, comparatively larger dorsal interarea, denser parvicostellae, 

 less well-developed ventral muscle field and dorsal platform. 



The specimen identified by Reed (1915: 13, pi. 3, fig. 3) as 

 Plectambonites cf. llandeiloensis (Davidson) from the Li-lu Forma- 

 tion (equivalent to the Upper Naungkangyi Group) at Li-lu, Northern 

 Shan States, has an undercut cardinal process, strongly elevated 

 bema and well-developed platform, and is here reassigned to 

 Leangella (Leangella) sp. Davidson's llandeiloensis itself has been 

 reassigned to Leptellina by Williams (1962: 164). A ventral valve, 

 the holotype of Reed's new species Leptelloidea {Leangella?) 

 lamellata (1936: 44, pi. 4. figs 22-23, 23a), from rocks correspond- 

 ing to the Naungkangyi Group at Taunggyi of the Southern Shan 

 States, is similar to L. (L.) minor in shell outline and convexity, but 

 differs from the latter in having fewer larger costellae, a more deeply 

 impressed and more elongate muscle field and a pair of strong 

 vascular dentalia. Since only a single ventral valve was illustrated, 

 we consider lamellata as generically indeterminable. Comparably, 

 Leptelloidea yeosinensis was described from Ye-o-sin, Southern 

 Shan States, by Reed (1932: 196, pi. 3, figs 3-6) and the well- 

 illustrated dorsal valves indicate that it is a leptellinid, but differing 

 from L. (L. ) minor in its lack of thin dorsal median septum anteriorly 

 and the presence of a bema. However, its detailed generic position is 

 not determinable. 



Several variations are observed in our specimens of L. {L.) minor, 

 including: ( 1 ) ventral convexity; most of the ventral valves are very 

 convex but some are more gentle: (2) geniculation; over 80% of the 

 dorsal valves have marked geniculation, while the remaining minor- 

 ity have much weaker or even absent geniculation; (3) ventral 

 muscle field; the dental plates are usually weak and enclose the 

 elevated muscle field in which the medial adductor scars are higher 

 than the lateral diductor scars. Sometimes the small teeth have no 

 supports and the slightly elevated diductor and adductor scars are 

 indistinguishable from each other; (4) cardinal process; the central 

 lobe is always well-developed, while the lateral lobes are often 

 absent; (5) platform; the presence of a platform is one of the main 

 characters of this genus, but it is variably developed and elevated in 

 L. (L.) minor, and occasionally it is even composed merely of a series 

 of discontinuous tubercles. 



DESCRIPTION. Exterior. Subquadrate to subcircular shell 6.9- 

 1 8.7mm long and 7. 7-23. 8mm wide with length/width ratio 0.71-1.0. 

 Lateral profile concavo-convex: dorsal valve slightly concave medio- 

 posteriorly with a small anterior geniculation; strongly convex 

 ventral valve particularly medially. Cardinal extremities round, 

 maximum width at about mid-length. Large flat apsacline ventral 

 interarea, small beak, only posterior one-third covered by small 

 arched pseudodeltidium; smaller hypercline dorsal interarea, 

 notothyrium completely occupied by cardinal process. 

 Multibranching costellae, equal in size, near the anterior margin, 

 about 4 per mm. Growth lines closely spaced, 12 per mm longitudi- 

 nally. 



Ventral interior. Stout triangular teeth without supports. Delthyrial 

 cavity with some secondary shell accumulation. Poorly-impressed 

 muscle field about one-third to two-fifths of shell length and width, 

 without apparent surrounding ridges, adductor and diductor scars 

 indistinguishable from each other. Variably-developed subperipheral 

 ridge extending posteriorly towards the hinge line and then medially 

 to the teeth lateral sides parallel to the hinge line. Vascular markings 

 saccate, a pair of vascular media originating in front of the muscle 

 field and extending forward with several branches. 



Dorsal interior. Small cardinalia 12-23% valve length and 21- 

 32% valve width; cardinal process usually simple but occasionally 

 trifid. median lobe elongate and strongly projecting ventrally and 

 posteriorly; thick and straight socket ridges divergent at 60-100°, 

 extending posteriorly and connecting with the lateral lobes of cardi- 

 nal process; the whole notothyrial cavity highly elevated and 

 thickened by secondary shell: large deep sockets open antero- 

 laterally. Slightly elevated quadrate muscle field with distinctive 

 bounding ridges, posterior pair of scars a little larger than the 

 anterior pair; thick and strong myophragm originating from the 

 notothyrial platform, becoming thinner and higher anteriorly, with 

 its acme just in front of the muscle field, merging into the platform. 

 Variably-developed quadrate platform slightly undercut and extend- 

 ing posteriorly to the hinge line. All the area outside the platform 

 geniculate dorsally. 



Measurements 





















L 



W 



LAV 



LI 



Ll/L 



Wl 



W1AV 



~ 



BC52 182. dorsal valve 



6.9 



7.7 



0.90 



1.3 



0.19 



2.1 



0.27 



75° 



BB37750, dorsal valve 



11.5 



16.1 



0.71 



2.6 



0.23 



3.6 



0.22 



60° 



BB37757. dorsal valve 



18.7 



23.8 



0.79 



2.3 



0.12 



5.0 



0.21 



100° 



BB37759. ventral valve 



8.7 



8.7 



1.0 



2.9 



0.33 



2.9 



0.33 



- 



BB37774, dorsal valve 



8.6 



10.4 



0.83 



1.6 



0.19 



3.3 



0.32 



89° 



SMA3128, ventral valve 



11.9 



12.3 



0.97 



5.2 



0.44 



5.0 



0.41 



- 



Genus BEKKERELLA Reed. 1936 



Bekkerella subcrateroides (Reed, 1906) PI. 3, figs 6-9 



1906 Orthis subcrateroides Reed: 63, pi. 4, figs 27-33. 



1915 Orthis subcrateroides Reed; Reed: 12. 



1936 Rafmesquina (Bekkerella) gentilis Reed: 38, pi. 4, fig. 14. 



Material and LOCALITIES. Nine dorsal internal, eight external, 

 10 ventral internal and four external moulds from the Naungkangyi 

 Group at Kunkaw (Localities YA42, YA50. 1 , YA256 andYA315.1); 

 one dorsal internal, two external and four ventral external moulds 

 from the Li-lu Formation (equivalent to the Upper Naungkangyi 

 Group) at Li-lu (about 1 1 km southeast of Longtawkno, Locality 

 YA630); one ventral valve (internal and external moulds) from the 

 Taungkyun Formation (equivalent to the Lower Naungkangyi Group) 

 at Li-lu (Locality YA 139); one ventral internal mould at Chaungzon, 

 and two dorsal internal moulds at Namyun, both from the 

 Naungkangyi Group; all in the Northern Shan States. 



DISCUSSION. Bekkerella appears endemic to Burma and is charac- 

 terized by undifferentiated fine radial ornamentation, a slightly 

 elevated and distinctive dorsal muscle field, a strong median septum 

 and a quadrate platform. Acculina and Shlyginia, also common in 

 Caradoc times, are similar to Bekkerella in dorsal interior, but they 

 both have parvicostellate ornamentation, and in addition Acculina 

 has a resupinate profile, well-developed pseudodeltidium and a 

 bilobed ventral muscle field with extended dental plates as bounding 

 ridges (Cocks & Rong 1989: 103). Shlyginia has a much larger 

 ventral muscle field within which adductor scars are enclosed by 

 diductor scars, and a small cardinal process seldom projecting 

 posterior to the hinge line. 



Reed (1936: 38) erected the subgenus Bekkerella within 

 Rafinesquina, with Orthis subcrateroides Reed (1906) from the 

 Naungkangyi Group at Chaungzon in the Northern Shan States as its 

 type species. The single ventral interior which he illustrated from the 

 Southern Shan States in 1936 has a muscle field which was overem- 

 phasised in the drawing (Reed 1936, pi. 4, fig. 14), since none of 



