16 



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



Measurements 



BB37736, dorsal valve 

 BB37737, ventral valve 

 BB37738, ventral valve 

 BB37739, dorsal valve 

 BB37743, ventral valve 



L 

 6.0 



5.2 

 9.1 

 4.5 

 6.2 



W 



7.0 

 11.5 

 6.8 

 8.1 



L/W 



0.74 

 0.79 

 0.66 

 0.77 



Wl 

 1.8 

 2.1 

 3.1 

 1.3 

 1.9 



Wl/W 



0.30 

 0.27 

 0.19 

 0.23 



N 

 16 

 15 

 19 

 16 

 15 



DISCUSSION. According to Williams' (1963: 352) emended di- 

 agnosis, Nicolella has a plano-convex shell, an ornament of simple 

 costae with a very few rarely developed costellae, strong teeth 

 supported by short receding dental plates, an elongately oval 

 ventral muscle field without any bounding ridges, and small 

 cardinalia with an elevated notothyrial platform. Both Reed's 

 (1936) illustrated specimen and our present material possess these 

 main characters. The specimen identified by Reed (1936: 20, pi. 

 1, fig. 15) as Orthis (Hesperorthis) cf. laurentina Billings is a 

 slightly distorted dorsal valve from the rocks corresponding to the 

 Naungkangyi Group in the Southern Shan States of the same age 

 as sylvatica. Its outline, ribbing and concentric growth lines are 

 similar to sylvatica, but without interiors is only question- 

 ably reassigned here to sylvatica. Although Orthis (Wattsella?) 

 pontilis (Reed 1936: 27, pi. 2, figs 5-7) has a similar locality and 

 horizon to sylvatica and the same exteriors as the latter, its large 

 ventral muscle field and apparent dorsal muscle bounding ridges 

 make us uncertain whether or not it is truly a junior synonym of 

 sylvatica. 



The type species of Nicolella, Orthis actoniae J. de C. Sowerby, 

 has been studied and discussed by several authors, such as Davidson 

 (1868: 252, pi. 36, figs 5-17), Williams (1963: 353, pi. 1 figs 15-19, 

 text-fig. 6) and Wright (1964: 165, pi. 2, figs 1-7, 10-1 1 ), all from 

 Caradoc material. It differs from sylvatica in having more widely 

 divergent brachiophores and in lacking the slight fold and sulcus. 

 Nicolella delicata (Xu, Rong & Liu 1974: 151, pi. 66, figs 28-30), 

 from the Shihtzupu Formation (early Caradoc) of Zunyi, Guizhou 

 Province, South China, can be distinguished from sylvatica by 

 having well-developed exopunctae and stronger dental plates en- 

 closing a more circular ventral muscle than sylvatica and might even 

 be attributable to Sulevorthis. O. (Eridorthis) liberalis (Reed 1936: 

 22, pi. 1, figs 3, 3a, 4) andO. (Eridorthis) kalavensis (Reed 1936: 23, 

 pi. 1, figs 1-2) are two new species named by Reed on the basis of 

 a ventral and a dorsal valve respectively. Because they are both from 

 the rocks corresponding to the Naungkangyi Group at Taungtala of 

 the Southern Shan States and have similar ribbing and shell sizes, we 

 think they might be the same species and, if so, liberalis would be the 

 senior synonym. It differs from sylvatica in having much coarser and 

 branching ribs and a sessile pseudospondylium in the ventral inte- 

 rior; however, with the minimal material available, its generic 

 attribution is doubtful. 



Nicolella sp. 



1936 Orthis (Nicolella) cf. actoniae Sowerby: Reed: 29, pi. 2, 

 fig. 9. 



Material and localities. Two dorsal and one ventral external 

 moulds from the Naungkangyi Group at Kunkaw; four dorsal and 

 two ventral external moulds from the Naungkangyi Group at 

 Chaungzon, both in the Northern Shan States; and one ventral 

 external mould from the rocks corresponding to the Naungkangyi 

 Group at Neyaungga in the Southern Shan States. 



DISCUSSION. Reed illustrated only exteriors, which makes further 

 identification unreliable, although the external morphology is simi- 

 lar to Nicolella actoniae J. de C. Sowerby from Shropshire. These 

 Burmese specimens differ from A', sylvatica in having nearly flat 

 dorsal valves and branching ribs (costellae) at two-thirds of their 

 shell length. 



Genus SAUCRORTHIS Xu. Rong & Liu. 1974 



Saucrorthis irravadica (Reed, 

 figs 1-5 



1906) PI. 1, fig. 11; PL 2, 



1906 



1915 



71932 



71936 



1936 



Orthis irravadica Reed: 62, pi. 4, figs 15-22. 



Orthis irravadica Reed; Reed: 1 1. 



Yeosinella consignata Reed: 193, pi. 3, figs 1-2. 



Orthis pustulifera Reed: 18, pi. 1, figs 7-14; pi. 2, fig. 3. 



Yeosinella consignata Reed; Reed: 30, pi. 4, fig. 11. 



Material and localities. One dorsal internal and external, and 

 one ventral external mould from the Naungkangyi Group at Namyun 

 (about 7 km southeast of Longtawkno), Northern Shan States; eight 

 ventral internal, nine external, five dorsal internal and eight external 

 moulds at Linwe (Locality AM77). three ventral internal and exter- 

 nal, and four dorsal internal and external moulds from the 

 Neyaungga- Ye-ngan area (Locality BA490), both from the Kinle 

 Siltstone Formation (equivalent to the Upper Naungkangyi Group) 

 in the Southern Shan States. 



Description. Exterior. Small semicircular shell 2.4-6. 6mm long, 

 3.3-9. lmm wide, with length/width ratio 0.70-0.88. Lateral profile 

 ventribiconvex with dorsal valve gently convex or even flat with a 

 median sulcus. Maximum width invariably along the hinge line. 

 Ventral interarea comparatively large, apsacline. delthyrium open. 

 Very small anacline dorsal interarea without chilidium. but most of 

 the notothyrium is occupied by the cardinal process. Ornament of 

 simple costae and fine concentric fila. Sharp costae generally 

 unbranching, coarser anteriorly, 18 on the dorsal valve and 15 on the 

 ventral valve irrespective of the shell size; no central costa on dorsal 

 valve. In a well-preserved specimen (PI. 2, fig. 5), minute but distinct 



PLATE 2 



Figs 1-5 Saucrorthis irravadica (Reed), la, lb, BB 37670, Neyaungga, Locality BA490, dorsal internal mould and latex cast, x 5. 2-5, Linwe, Locality 



AM77; 2, BB 37702, ventral internal mould, x 5; 3, BC 5241 1, latex cast of dorsal exterior, x 4; 4a-c, BB 37668, 4a, dorsal internal mould, x 5, and 4b, 



4c, latex casts of dorsal exterior and interior, x 4; 5, BB 37695, dorsal external mould showing sparse exopunctae, x 12. 

 Figs 6-10 Onniella chaungzonensis (Reed). 6, BB 37676, Neyaungga, Locality BA490, latex cast of ventral exterior, x 4. 7-10, Ta-Pangtawng, Locality 



YA454.1. 7a, 7b, BB 37731, latex cast of dorsal exterior, and dorsal internal mould, x 4. 8, BC 52412, ventral internal mould, x 8. 9, BC 52413, latex 



cast of dorsal exterior with ventral interarea, x 8. 10a, 10b, BC 52416, latex cast and internal mould of dorsal interior with a ventral valve (BC 52415) of 



Nicolella sylvatica (Reed), x 4. 

 Figs 11-12 Indeterminate clitambonitid. Kunkaw. 11, BB 37777, Locality YA40. 1 , ventral internal mould, x 5. 12, BB 37771, Locality YA39, ventral 



internal mould, x 3. 

 Figs 13-14 Leptellina (Leptellina) minor sp. nov. Linwe, Locality AM78. 13a, 13b, BB 37633, dorsal internal mould, and latex cast, x 4. 14a-c. BB 37643, 



ventral internal mould, latex cast and posterior view, x 5. 



