CARADOC BRACHIOPODS FROM THE SHAN STATES 



123 



Reed's illustrated ventral valve muscle fields ofsubcrateroid.es is so 

 elongate and divergent. All our present specimens are also from the 

 Naungkangyi Group from several localities in the Northern Shan 

 States and are identical tosubcrateroid.es. Some variations observed 

 in this material are: (1) dorsal concavity; most dorsal valves are 

 slightly concave or nearly flat medial-posteriorly, with a small but 

 strong geniculation anterior to the platform, but there are a few 

 individuals with an evenly concave dorsal valve and no geniculation 

 (e.g. PI. 3, fig. 6); (2) cardinal process: lateral lobes are usually 

 absent, but they are present in a few specimens and continuous 

 antero-laterally with the straight socket ridges; (3) platform; most 

 dorsal valves have a well-developed and slightly undercut platform, 

 but a few specimens, particularly juveniles, have a very weak 

 platform. 



Subfamily PALAEOSTROPHOMENINAE Cocks & Rong, 1989 

 Genus ISHIMIA Nikitin, 1974 



Ishimia subdeltoidea (Reed, 1906) 



PI. 3, fig. 10; PI. 4, figs 1-7 



1906 Rafinesquina subdeltoidea Reed: 52-53, pi. 5, figs 1-8. 

 1936 Rafinesquina (Kjaerina) cf. felix Reed; Reed: 37, pi. 4, 

 fig. 1. 



Material and localities. Eight dorsal internal, six external, 

 eight ventral internal and three external moulds at Tawmawgon 

 (about 30 km north of Kyaukme); three dorsal internal, one ventral 

 internal and one external moulds at Kunkaw (Localities YA45.1, 

 YA3 15), both from the Naungkangyi Group; two dorsal internal and 

 external moulds from the Li-lu Formation (equivalent to the Upper 

 Naungkangyi Group) at Ta-Pangtawng (about 10 km east of 

 Longtawkno); all in the Northern Shan States. Two dorsal internal 

 moulds from the equivalents of the Upper Naungkangyi Group at 

 Linwe (Locality AM77), Southern Shan States. 



TYPES. Lectotype, here selected, the original of Reed 1906. pi. 5, 

 fig. 4, a dorsal internal mould from Tawmawgon. Northern Shan 

 States. Indian Geological Survey Museum, Calcutta. 



Description. Exterior. Lateral profile concavo-convex; dorsal 

 valve flat or slightly concave medial-posteriorly, with variably 

 developed dorsal geniculation. some strong and nearly perpendicu- 

 lar; ventral valve convex, with convexity increasing anterior to the 

 dorsal valve geniculation. Cardinal extremities usually acute, maxi- 

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

 interarea; ventral beak slightly curved; delthyrium only about one- 

 quarter covered by pseudodeltidium. Smaller hypercline dorsal 

 interarea with a well-developed arched chilidium. Ornament of 

 coarse parvicostellae. unevenly distributed, about 3-4 per mm near 

 the margin; only 1-2 finer costellae between two coarser ones. 

 Growth lines well-preserved on the geniculation, 8-9 per mm; 

 several concentric comae often present postero-laterally. 



Ventral interior. Small triangular or wedge-like teeth without 

 supports. Weakly-impressed muscle field with no bounding ridges; 

 antero-median pair of diductor scars much larger than the postero- 

 lateral pair and with a short and weak myophragm, small adductor 

 scars in the posterior centre of the muscle field. Vascular markings 

 leminiscate; vascular media originating from the inner sides of the 

 anterior end of the muscle field, and vascular myaria starting between 

 the two pairs of diductor scars. 



Dorsal interior. Cardinalia about one-fourth shell length and 

 width; very high, thin plate-like cardinal process median lobe pro- 



jecting ventrally, lateral lobes often absent; straight socket ridges 

 separated from the median lobe, divergent at about 75-100°, extend- 

 ing antero-laterally far beyond the sockets, showing thin plate-like 

 crura: the cardinal area elevated by the deposition of secondary 

 shell; deep and narrow or round sockets open or with thick and low 

 bounding ridges antero-laterally. Well-impressed elongate oval mus- 

 cle field including a smaller anterior pair and larger posterior pair of 

 adductor scars, low bounding ridges often absent; thick and high 

 myophragm originating from the notothyrial platform and extending 

 forward to become thinner and higher, reaching its acme at the 

 anterior end of the muscle field and continuous anteriorly with the 

 thin median septum which ends before or merges into the platform. 

 Quadrate platform slightly elevated, extending posteriorly to the 

 hinge line. The geniculation is immediately anterior of the platform. 



Discussion. Most of our present material was collected from 

 Reed's type locality of subdeltoidea. Tawmawgon in the Northern 

 Shan States, and is identical to Reed's illustrated specimens. The 

 distorted dorsal interior, from rockscorresponding to the Naungkangyi 

 Group at Hpongyi Kyaung in the Southern Shan States and identified 

 by Reed as Rafinesquina (Kjaerina) cf. felix. is basically similar to 

 some of our specimens and is thus assigned to subdeltoidea here. The 

 true felix of Reed ( 1917) is a quite different strophomenoid from the 

 middle Ashgill of Girvan, Scotland (Cocks 1978). When Nikitin 

 ( 1 974: 59) established/?/; imia from the Middle Ordovician of Central 

 Kazakhstan, he recognised four species./, humilis (Nikitin 1974: 63; 

 pi. 6, figs 1-3). from the Yerkebidaik Horizon (middle Caradoc) of 

 Chingiz in Central Kazakhstan, lacks geniculation, but otherwise is 

 the most similar to subdeltoidea, although it differs in having finer 

 costellae and a more elongate bilobed ventral muscle field. The type 

 species, /. ishimensis (Nikitin 1974: 61, pi. 5, figs 10-16) from the 

 Karakan Horizon (late Llanvirn) of Kupriyanovka in Central 

 Kazakhstan, can be distinguished in having finer and more differenti- 

 ated costellae. larger ventral adductor scars, and stronger dorsal 

 bounding ridges, median septum and platform. 



Some variations within the dorsal interior observed from our 

 material of/, subdeltoidea are: ( 1 ) cardinal process; most specimens 

 have only a strong single cardinal process lobe, but a few develop a 

 pair of small lateral lobes, making the cardinal process trifid and 

 hence the genus is appropriately placed within the Leptellinidae; (2) 

 sockets; the antero-lateral sides are sometimes open, while a few 

 specimens have low bounding ridges; (3) median septum; this 

 usually ends before the platform, but in a few specimens it merges 

 anteriorly with the platform. 



Family SOWERBYELLIDAE Opik, 1930 

 Subfamily PTYCHOGLYPTINAE Cooper. 1956 



Genus PTYCHOGLYPTUS Willard, 1928 



Ptychoglyptus? shanensis Reed, 1932 



1932 Ptychoglyptus shanensis Reed: 195, pi. 3. fig. 15. 

 1936 Ptychoglyptus shanensis Reed; Reed: 37. 



Discussion. Reed (1932) named the species Ptychoglyptus 

 shanensis from rocks corresponding to the Naungkangyi Group at 

 Ye-o-sin in the Southern Shan States, on the basis of a single 

 specimen showing distinctive zigzag rugae interrupted by costae. 

 Although it may be a Ptychoglyptus, this is not certain until interims 

 are discovered, since very similar ornament can be found on some 

 strophomenoid rafinesquinds such as Pentlandina. Thus the generic 

 attribution is queried here. 



