ORDOVICIAN BRACHIOPODA 125 



Oxoplecia cf. nantensis MacGregor 

 (PI. 20, figs. 9-13) 

 1961 Oxoplecia nantensis MacGregor : 196. 



Diagnosis. Subcircular, subequally biconvex Oxoplecia with a variably 

 developed dorsal fold over half as wide as the brachial valve length, and fine radial 

 ornament consisting of ribs averaging 0-3 mm in wavelength, 5 mm antero-medially 

 of dorsal umbo. 



Description. Small, subequally biconvex Oxoplecia with obtuse cardinal angles 

 and a hinge-line about three-quarters the maximum width of the shell ; pedicle 

 valve 23% as deep as long with rounded variably defined sulcus, brachial valve 84% 

 as long as wide and 29% as deep as long with a broadly rounded fold 57% as wide 

 as the brachial valve is long ; ventral interarea curved apsacline, pseudodeltidium 

 with narrow median fold, dorsal interarea linear ; up to 10 rounded branching ribs, 

 usually not including a median costa and with a mean wavelength of 0-31 mm, 

 5 mm anterior of the umbones, occur on the folds and 10 to 12 ribs on the flanks of 

 5 brachial valves less than 10 mm long ; concentric ornamentation of overlapping 

 lamellae numbering about 7 between 5 and 6 mm anterior of the dorsal umbo. 



Pedicle passage 5% as long as the pedicle valve, rarely with incipient traces of 

 pedicle tube, small teeth supported by acutely divergent dental plates extending 

 forward for 11% the length of the valve ; muscle field unknown. 



Cardinalia consisting of deeply bilobed cardinal process and short brachiophores, 

 simple sockets arranged parallel to the hinge-line and extending laterally for almost 

 one-quarter the length of the valve. 



Figured material. 



length width (mm) 

 External and internal moulds of pedicle valve (BB 35320a, b) 8-0 8-5 



Incomplete internal mould of brachial valve (BB 35321) 6-0 8-0 



External and internal moulds of brachial valve (BB 35322a, b) 8-5 9-5 



Incomplete internal mould of brachial valve (BB 35323) - 



Horizon and localities. Meadowtown Beds : BB 35322 from exposures along 

 the lane to Lower Ridge from bench mark 754, Little Weston (Grid Ref. SO 293984) ; 

 the remaining specimens from temporary exposures at the side of a cart-track near 

 Waitchley (Grid Ref. SJ 311018). 



Discussion. Oxoplecia is fairly widely distributed throughout the Lower 

 Llandeilo rocks of Wales and Shropshire but is never common in any one locality so 

 that data about its morphological variability are not abundant. It is, however, 

 immediately distinguishable from penecontemporaneous Oxoplecia from other 

 regions in the variable development of the fold and sulcus and especially in the 

 fineness of the radial ornamentation. This is true even of the most closely com- 

 parable American species, the Chazyan Oxoplecia costellata (Cooper 1956 : 540), 

 which further differs in the late development of fold and sulcus. The few Oxoplecia 

 species reported from Ordovician successions of Eurasia also differ in other attri- 

 butes. Oxoplecia sibirica, from the Mangaseya Stage of the Siberian platform 



