ORDOVICIAN BRACHIOPODA 73 



valve, the absence of a dorsal sulcus, the relative fineness of the costellate ornamenta- 

 tion, the variable development of a simple cardinal process and the occurrence of 

 hollow embayments along the crests of the costae. Indeed, in respect of the last 

 two features, the new species shows some affinity with Desmorthis Ulrich & Cooper 

 1936, and in view of the essential homogeneity of the American Plectorthis which 

 suggests that they constitute a closely related species group, the Shropshire specimens 

 may represent a stock with only remote ancestral links with their American con- 

 temporaries. 



Plectorthis sp. 

 (PI. 11, figs. 12, 15) 



A complete internal mould and a fragment of the external mould (BB 35466a, b) of 

 an orthacean pedicle valve, collected from Aldress Shales exposed in a bank of Ox 

 Wood Dingle at the south-west corner of Ox Wood, a few yards north of the Rorring- 

 ton-Wotherton road (Grid Ref. SJ 290007), have been provisionally identified as 

 Plectorthis. The valve, which was about 13 mm long and 14-5 mm wide, was subcircular 

 in outline with obtusely rounded cardinal angles and just over one-sixth as deep as 

 long becoming flatter antero-medially. The surface was ornamented by micro- 

 scopic concentric lamellae and 24 ribs which must have arisen at or near the umbo 

 except for a few secondary costellae in the lateral areas. The ribs were rounded 

 with a wavelength of o«6 mm, 10 mm anterior of the umbo, and were separated by 

 interspaces about 0-5 mm wide. The interarea was short, curved, and apsacline, 

 the delthyrium open, and the small teeth were supported by widely divergent dental 

 plates extending anteriorly for less than one-sixth the length of the valve. No 

 identifiable muscle scars were impressed on the floor of the valve. 



In the absence of impressions of the brachial valve, no valid comparisons can be 

 drawn with described Plectorthis. The pedicle valve, however, is immediately 

 distinguishable from that of P. whitteryensis sp. nov. in its subcircular rather than 

 semi-elliptical outline, its significantly fewer ribs which are, moreover, rounded not 

 flattened, and its widely divergent dental plates. 



DESMORTHIS Ulrich & Cooper 1936 



Desmorthis ? sp. nov. 



(PI. 12, figs. 2-4, 8) 



Incomplete external and internal moulds of a brachial valve (BB 35337a, b), from 

 exposures in a runnel to the east of the footpath to Blakemoorflat at the head of 

 Mytton's Beach (Grid Ref. SJ 373006), are the sole representatives in the Mytton 

 Flags of a highly distinctive plectorthid genus. The valve which was 7 mm long, 

 semi-oval in outline, rectimarginate and gently convex medially with flattened 

 lateral slopes, was ornamented by strong, sharply rounded hollow costellae number- 

 ing 2 per mm, 5 mm antero-medially of the umbo, and a subdued concentric orna- 

 mentation of faint lamellae. The planar anacline interarea was about one-tenth as 



