CARADOC BRACHIOPODS FROM THE SHAN STATES 



119 



Description. Exterior. Small transverse shell 2.4-5.4mm long 

 and 2.7-6.4mm wide with length/width ratio 0.75-0.89. Lateral 

 profile unequally biconvex; gently convex dorsal valve with a con- 

 spicuous sulcus originating from the umbo, much deeper ventral 

 valve with strongest convexity along the hinge line. Maximum width 

 near the shell midlength. Ventral beak small; slightly curved apsacline 

 interarea; open delthyrium; narrow anacline dorsal interarea; with- 

 out chilidium but notothyrium occupied by strong cardinal process 

 lobes (PI. 2, fig. 10a). Ornament of densely populated costellae, 

 branching medially and laterally on dorsal valve and laterally on 

 ventral valve three times: firstly at one-quarter shell length, secondly 

 at one-third length and finally at two-thirds length. No median rib on 

 dorsal valve, but a deep and narrow groove along the median line 

 with a pair of weak costellae on both sides starting at one-quarter of 

 the shell length. Concentric growth lines dense and even over the 

 whole shell. One or two stronger growth lines common near the 

 anterior margin. Endopunctate shell. 



Ventral interior. Strong teeth supported by a pair of subparallel, 

 thick and short dental plates. Poorly impressed cordate muscle 

 field about 30% of the length and width; slightly elevated central 

 adductor scars not enclosed by diductor scars anteriorly. Strong 

 crenulations near the margins of both valves about one-quarter of 

 the length, which form broad, flat ridges separated by narrow 

 deep grooves. 



Dorsal interior. Strong and erect cardinal process limited to the 

 posterior part of the notothyrial cavity, well-developed myophore 

 fissured centrally. Robust brachiophores triangular at their bases and 

 highly projecting mainly ventrally and slightly anteriorly; short and 

 slightly divergent stout fulcral plates variably developed (strong in 

 PI. 2, fig. 10). No apparent notothyrial platform, but weak elevation 

 of notothyrial cavity often developed. Well impressed rectangular 

 muscle field just in front of the cardinalia, extending to more than 

 60% of the length and about 40% of the width, with low and wide 

 bounding ridges and myophragm; two pairs of adductor scars, with 

 the anterior pair of scars larger than the posterior pair. Low and wide 

 median ridge extending to the anterior margin. 



Measurements 



















L 



W 



LAV 



L, 



L, 



W, 



W, 



BB37703, dorsal valve 



5.2 



5.8 



0.89 



0.9 



- 



1.3 



- 



BB37731, dorsal valve 



5.4 



6.3 



0.86 



1.4 



3.1 



2.0 



2.5 



BB37736, ventral valve 



2.4 



2.7 



0.89 



0.8 



- 



0.7 



- 



BB37738, dorsal valve 



4.8 



6.4 



0.75 



1.0 



3.4 



1.9 



2.7 



DISCUSSION. Bancroft (1928: 55) established Onniella for small 

 dalmanellids with a transverse shell, small beaks, dorsal sulcus and 

 no ventral pallial markings. Later (Bancroft, 1945: 21 1 ), he further 

 summarized the main and distinguishing characters of Onniella as a 

 small dalmanellid with Resserella-tike crural plates, unequal-sized 

 dorsal muscle scars, feebly-developed ventral muscle field, and 

 without apparent pallial markings.According to Williams &Wrighf s 

 (1963) detailed revision, Dalmanella, externally somewhat close to 

 Onniella, differs from the latter in having small fulcral plates, 

 convergent brachiophore supports, comparatively smaller dorsal 

 muscle scars and a more elongate ventral muscle field. Hurst (1979) 

 has discussed and redefined the various species of Onniella from the 

 type Caradoc area of Shropshire, including O. broeggeri Bancroft, 

 the type species. According to his convincing discussion, the differ- 

 entiation of species within Onniella should be on ribbing and some 

 aspects of the interiors, especially the shape of the ventral muscle 

 field, rather than only on their ribbing patterns as Bancroft believed. 

 The present material from Burma lacks fulcral plates in most 

 specimens, has slightly divergent brachiophore supports, the ventral 

 adductor scars are not enclosed by diductor scars and has rectangular 



dorsal muscle fields with larger anterior pair of scars; it is therefore 

 assigned to Onniella. 



All the specimens identified as Orthis (Dalmanella?) 

 chaungzonensis by Reed (1906: 61, pi. 4, figs 7-14) are from the 

 Naungkangyi Group at Chaungzon, Northern Shan States, and have 

 no essential differences from our material except for the equal-sized 

 pairs of dorsal adductor scars shown in his figure 9, plate 4, which 

 may have been overemphasised in Reed's drawing. The two exteri- 

 ors identified as Orthis (Dalmanella) testudinaria by Reed (1906: 

 60. pi. 4, figs 25-26), from the same locality and horizon as chaung- 

 zonensis, have the same ribbing as the latter. Orthis (Dalmanella) 

 testudinaria shanensis (Reed, 1915: 9, pi. 2, figs 6-11), from the 

 Hwe Mawng Formation (equivalent to the Upper Naungkangyi 

 Group) at Hkawnhkok, Northern Shan States, also has the same 

 characters as chaungzonensis, except for two ventral interiors (Reed, 

 1915: pi. 2, figs 8, 11) which may be attributable to Dalmanella 

 rather than Onniella, because the general shape of the ventral muscle 

 field and the adductor and diductor scars are identical to that of the 

 real D. testudinaria from the Baltic. 



The Orthis (Dalmanella) elegantula of Reed (1906: 60, pi. 4, figs 

 23-24) was based on two distorted specimens from the Naungkangyi 

 Group at Taungkyun, Northern Shan States, and differs from 

 chaungzonensis in shell outline, ribbing and cardinalia but cannot be 

 identified with certainty here. Orthis (Dalmanella) sincluumgensis 

 (Reed 1936: 28, pi. 2, figs 12— 15a) was named from the rocks 

 corresponding to the Naungkangyi Group at Taungbu, Southern 

 Shan States, and can be distinguished from chaungzonensis by much 

 denser costellae, different ribbing style and a larger dorsal muscle 

 field consisting of two pairs of equal-sized adductor scars, and may 

 be a draboviid. The type species of Onniella, O. broeggeri (Bancroft 

 1928: 56, pi. 2, figs 1-5) from the Onny Shale Formation of 

 Shropshire, differs from chaungzonensis in having coarser costellae 

 and a smaller dorsal muscle field. 



Superfamily CLITAMBONITOIDEA Winchell & Schuchert, 1893 

 Family CLITAMBONITIDAE Winchell & Schuchert, 1893 



Indet. clitambonitid 



PI. 2, figs 11-12 



Material and locality. Four ventral internal and two external 

 moulds from the Naungkangyi Group at Kunkaw (Locality YA40) in 

 the Kyaukme-Longtawkno area. Northern Shan States. 



Description. Transverse elliptical shell 5.1- 10.2mm long and 

 7. 0-13. 8mm wide with length/width ratio 0.73-0.92. Variably con- 

 vex ventral valve with a shallow and narrow sulcus originating in 

 front of the umbo; apsacline interarea with a large and open 

 delthyrium. Maximum width along the straight hinge line or slightly 

 in front of it. Ornament of multicostellae, 3^4- per mm near the 

 anterior margin. Teeth small; short and shallow spondylium sup- 

 ported by a weak median septum. 



Discussion. One of the ventral internal moulds is clearly a 

 clitambonitid, based on the spondylium supported by the short 

 median septum, but it is uncertain whether or not a pseudodeltidium 

 or chilidium is present and thus to which subfamily it should be 

 attributed. The Clitambonites cf. squamata Pahlen recognised by 

 Reed (1906: 66, pi. 5, fig. 14), from the Naungkangyi Group at 

 Kunlein. Northern Shan States, and the Clitambonites cf. ascendens 

 Pander identified by Reed (1936: 31. pi. 3, fig. 14). from rocks 

 corresponding to the Naungkangyi Group at Nam Wabya. Southern 

 Shan States, are both based on single specimens which are very 



