ORDOVICIAN BRACHIOPODA 103 



brachial valve 76% as long as wide and 12% as deep as long with a strong median 

 sulcus about two-thirds as wide as the valve length, flanked by flattened lateral 

 areas ; ventral and dorsal interareas planar, apsacline and anacline respectively with 

 open delthyrium and notothyrium ; radial ornamentation fascicostellate with sub- 

 angular ribs branching internally and numbering 3, 4 and 5 per mm, 2 mm antero- 

 medially of the umbones of 1, 2 and 1 brachial valves. 



Teeth supported by divergent dental plates extending forward for 22% of the 

 length of the valve ; ventral muscle scar bilobed, about as wide as long, with sub- 

 triangular diductor impressions extending forward for 40% the length of the valve 

 but not enclosing the shorter median adductor scar. 



Cardinal process simple, blade-like, brachiophores narrowly divergent with sub- 

 parallel bases extending forward for 18% of the length of the brachial valve and 

 deeply separated from well-developed fulcral plates defining splayed sockets ; 

 adductor scars lightly impressed on either side of the wide median ridge except 

 posteriorly where they are inserted as a pair of shallow hollows into the notothyrial 

 platform. 



Figured material. 



length width (mm) 

 External and internal moulds of pedicle valve (BB 35431a, b) 6«o 6*5 



Internal mould of brachial valve (BB 35432) 3-5 4-0 



Internal mould of brachial valve (BB 35433) 4-0 



Horizons and localities. Spy Wood Grit : BB 35431 from exposures on top 

 of ridge 1440 yds NNE of Rorrington bench mark 599 (Grid Ref. SJ 303018) : 

 BB 35432 from exposures 1100 yds north-east of Rorrington (Grid Ref. SJ 303015). 

 Aldress Shales : BB 35433 from exposures in bank of Ox Wood Dingle at south-west 

 corner of Ox Wood a few yards north of the Rorrington-Wotherton road (Grid Ref. 

 SJ 290007). 



Discussion. A small number of moulds of Horderleyella, mainly those of brachial 

 valves, have been recovered from the Spy Wood Grit and Aldress Shales. They 

 appear to be conspecific with H. plicata Bancroft (1928 : 186 ; 1945 : 236) from the 

 Costonian and Harnagian successions of E. Shropshire, especially in the radial orna- 

 mentation and the shape and internal morphology of the brachial valve. The differ- 

 entiation of the cardinal process of H. plicata into myophore and shaft is probably 

 not a diagnostic feature in view of the much smaller size of the Shelve specimens. 

 Such differences in shell size may also account for the relative delicacy of the car- 

 dinalia of H. subcarinata MacGregor (1961 : 193) from the Upper Llandeilo of the 

 Berwyn Hills which may also be conspecific with H. plicata. 



In contrast to the close similarity of the brachial valves, the Shelve pedicle valves 

 may prove to be relatively deeper and the ventral muscle scar wider and shorter, 

 being only 79% as wide as long and 31% of the length of three pedicle valves of H. 

 plicata. Larger collections, however, might indicate that these differences fall within 

 the range of variability of H. plicata and, provisionally at least, the Shelve material 

 can be identified as that species. 



