114 



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



Superfamily ORTHOIDEA Woodward, 1852 



Family PLAESIOMYIDAE Schuchert, 1913 



Subfamily PLAESIOMYINAE Schuchert, 1913 



Genus PLAESIOMYS Hall & Clarke, 1892 



tral muscle field. Plaesiomys porcata (M'Coy), from the Portrane 

 Limestone (Caradoc), Ireland (Wright 1964: 187, pi. 4, figs 1- 

 12), is different from taungtalensis in having an 'isolated' cardinal 

 process (without any notothyrial platform). 



Plaesiomys taungtalensis (Reed, 1936) 



PI. l.figs 1-5 



Family PRODUCTORTHIDAE Schuchert & Cooper, 1931 

 Subfamily PRODUCTORTHINAE Schuchert & Cooper, 1931 



1906 Orihis (Dinorthis) flabellulum Sowerby; Reed: 62, pi. 4, 



figs 4-6. 

 1936 Orthis (Glyptorthis) taungtalensis Reed: 24, pi. 2, figs 10, 



10a, 11. 



Material and localities. One dorsal valve (external and inter- 

 nal moulds) from Chaungzon, longitude 96°52'E, latitude 22°21'N; 

 one dorsal external mould from Naungkangale; and two dorsal 

 valves (external and internal moulds) from Pangmaklang (about 20 

 km northeast of Kunkaw. Locality YA 365, longitude 97°16'E, 

 latitude 22°42'N); all from the Naungkangyi Group of the Northern 

 Shan States. 



Discussion. Plaesiomys and Dinorthis are both large orthoids 

 with relatively small cardinalia and long and elevated sub-triangular 

 to subpentagonal ventral muscle fields (Wright 1 964), but the former 

 has multibranching costellae and the latter has simple costae. All the 

 present specimens have branching costellae and small but well- 

 developed cardinalia (about one quarter shell length) with a highly 

 projecting plate-like cardinal process which is limited to the vari- 

 ably-developed notothyrial platform, and so they are included in the 

 genus Plaesiomys, although no ventral valves are available. 



Reed (1906) described and illustrated three specimens from the 

 Naungkangyi Group at Chaungzon as Sowerby's species flabellulum 

 under the subgenus Dinorthis, which are the same species as our 

 material in ribbing and dorsal interior. However, the true flabellulum 

 (Williams 1963: 363, pi. 3, figs 1^4) has simple costae which only 

 exceptionally branch. Orthis (Glyptorthis) taungtalensis (Reed 1936) 

 was named from the Naungkangyi Series of Taungtala, Southern 

 Shan States, and has branching ornamentation, a very convex dorsal 

 valve and small cardinalia limited to the notothyrial platform, which 

 are all present in our material and typical of Plaesiomys. Orthis 

 (Dinorthis) porcata birmanica (Reed 1915: 10, pi. 2. figs 12-13) is 

 another species from the Upper Naungkangyi Group at Ta-Pangtawng 

 of the Northern Shan States which should be reassigned to Plaesio- 

 mys, but it differs from taungtalensis (including Reed's specimens 

 of 'flabellulum' ) in having a more circular shell, denser costellae and 

 a more elongate and anteriorly bilobed ventral muscle field. 



The type species of Plaesiomys, Orthis subquadrata (Hall 1847: 

 126, pi. 32A, figs la-o) from the Richmondian (late Caradoc and 

 early Ashgill) of Ohio, USA, differs from taungtalensis in having 

 much denser costellae and a much stronger and larger crenulated 

 myophore. Plaesiomys robusta and Plaesiomys multiplicata, both 

 from the late Caradoc of Glyn Ceiriog, Wales (Bancroft 1945), are 

 very similar to taungtalensis, but can be distinguished by their 

 more rounded shell, denser costallae, and different shape of ven- 



Genus NICOLELLA Reed, 1917 



Nicolella sylvatica (Reed, 1936) 



PI. 1, figs 6-10 



1936 Orthis (Glossorthis) sylvatica Reed: 21, pi. 2. figs 1-2. 

 71936 Orthis (Hesperorthis) cf. laurentina Billings; Reed: 20, pi. 



1, fig. 15. 

 71936 Orthis (Wattsella?) pontilis Reed: 27, pi. 2, figs 5-7. 



Material and locality. Four ventral external, six internal, and 

 two dorsal valves (internal and external moulds) from the Li-lu 

 Formation (upper part of Naungkangyi Group) at Ta-Pangtawng 

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

 96°23'E, latitude 22°58'N) in the Northern Shan States. 



Description. Exterior. Semicircular shell 4.5-9. lmm long and 

 6.8-1 1.5mm wide with the length/width ratio 0.66-0.79. Lateral 

 profile ventri-biconvex, gently convex dorsal valve with a clear 

 sulcus originating from the umbo. Maximum width along the hinge 

 line which extends laterally, forming a small ear. Curved and small 

 ventral interarea apsacline; narrow dorsal interarea anacline. Ante- 

 rior commissure slightly sulcate. Ornament of 15-19 simple costae 

 occasionally with some branching in the postero-lateral parts. No 

 median costa on dorsal valve but a pair of comparatively weaker 

 costae appear beside the median groove. Closely-spaced concentric 

 growth lines well-developed on some specimens on the anterior one- 

 third of the shell and showing several typically productorthid 

 imbrications. No exopunctae observed. 



Ventral interior. Strong triangular teeth supported by thin, short 

 and subparallel dental plates. Small, elongately oval and weakly- 

 impressed muscle field without any apparent anterior or antero-lateral 

 bounding ridges; the adductor and diductor scars are not distinguish- 

 able. Strong ribs on the surface of the shell, reflected on the internal 

 surface of both valves in a series of corresponding deep and narrow 

 radial grooves which include the muscle field; the intervals between 

 grooves are almost three times as wide as the groove. No vascular 

 markings seen. 



Dorsal interior. Small cardinalia about one-fifth shell length and 

 width; strong cardinal process with a swollen myophore. separated 

 from the brachiophores, occupying the whole notothyrial platform 

 which is slightly elevated; brachiophores triangular at their bases, 

 projecting highly anteriorly and ventrally at about 75° to one another; 

 weak brachiophore supports extend medially and meet at the median 

 ridge to form a low and wide ridge parallel to the hinge line. Poorly 

 impressed muscle field with a low and wide myophragm extending 

 to the anterior margin. 



PLATE 1 



Figs 1-5 Plaesiomys taungtalensis (Reed). 1, BC 52159, Kunkaw, Locality YA43, dorsal internal mould, x 2. 2, BB 37726, Pangmaklang, Locality YA365, 

 latex cast of a dorsal internal mould showing the cardinalia, x 4. 3, SMA 3 132, Chaungzon, latex cast of a partial dorsal external mould, x 2. 4a, 4b, 

 SMA 3131, Chaungzon, dorsal internal mould and latex cast, x 2. 5, BB 37724, Pangmaklang. Locality YA365, dorsal internal mould, x 2.5. 



Figs 6-10 Nicolella sylvatica (Reed). 6-9, Ta-Pangtawng, Locality YA454.1 . 6a-c, BB 37738, latex casts of ventral exterior and interior, and ventral 

 internal mould, x 3. 7a, 7b. BB 37736, dorsal internal mould and latex cast, x 4. 8a-c, BB 37739, 8a, latex cast of dorsal exterior with a ventral exterior 

 at the top right (BC 52414), x 4; 8b, 8c, latex cast and internal mould of dorsal interior, x 4. 9a, 9b, BC 52414, latex cast and internal mould of ventral 

 interior, x 4. 10a, 10b, BB37682, Neyaungga. Locality BA490, latex cast and internal mould of ventral interior, x 4. 



Fig. 11 Saucrorthis irravadica (Reed). BB37705, Linwe, Locality AM77, latex cast and internal mould of ventral interior, x 4. 



