ORDOVICIAN BRACHIOPODA 135 



with strong submedian septa extending anteriorly for 69% of the length of the 

 brachial valve. 



Description. Semicircular Sowerbyella with obtusely rounded cardinal angles in 

 adult shells, gently concave brachial valve 50% as long as wide (with a range of 47% 

 to 52% for 5 valves), pedicle valve with a mean depth relative to length of 26% for 

 6 valves (range 18% to 31%) with steeply sloping sides ; ventral and dorsal interareas 

 apsacline and anacline respectively with a supra-apical foramen and a small pseudo- 

 deltidium and complementary chilidium ; radial ornamentation unequally parvi- 

 costellate with counts of 7, 8, 9, 10 and 11 ribs per mm, 2 mm antero-medially of the 

 umbones of 1, 2, 4, 3 and 1 brachial valves respectively, with sectors delineated by 

 thickened costellae having a mean width of 0-93 mm (for 8 estimates with a range 

 of 0-7 to 1-2 mm) ; up to 9 pairs of rugae with a modal wavelength of 0-3 mm occur 

 in the postero-lateral areas at acute angles to the hinge-line. 



Teeth small, dental plates obsolescent, cordate ventral muscle field 65% as long 

 as wide and extending anteriorly for an average of 30% of the length of 6 pedicle 

 valves (range 24% to 38%) ; adductor scars deeply inserted in a pair of hollows in the 

 umbonal secondary shell and divided by a fine median ridge ; diductor scars splayed 

 anteriorly and divided by vascula media forming part of a lemniscate mantle canal 

 system. 



Cardinalia consisting of a median cardinal process fused with chilidial plates and 

 widely divergent socket plates extending anteriorly for 35% of their lateral spread 

 and for an average of 14% of the length of 3 brachial valves ; notothyrial platform 

 poorly developed ; lophophore and muscle supports normally consisting of a semi- 

 circular platform with strong septa bounding a median cleft extending anteriorly 

 for 69% of the length of brachial valves (range 60% to 75%). 



Figured material. 



length width (mm) 

 Internal mould of pedicle valve (BB 35555) 87 14-0 



External and internal moulds of brachial valve (BB 35556a, b) 6-3 12-0 

 Pedicle valve (BB 35557) 9' 13*5 



Horizon and localities. Whittery Shales : BB 35555 from exposures in the 

 lower part of the west bank of the River Camlad, 60 yds north-east of Marrington 

 Farm (Grid Ref. SO 272970) ; BB 35556 from Whittery Quarry at the south end of 

 Whittery Wood, near Chirbury (Grid Ref. SO 275981) ; BB 35557 from exposures 

 in the stream at the north end of Spring Coppice 865 yds south-east of Hockleton 

 Bridge (Grid Ref. SO 279997). 



Discussion. Species of Sowerbyella found in the Shelve succession belong to two 

 distinct supra-specific groups which may eventually prove to be worth recognizing 

 taxonomically in the manner proposed by Roomusoks (1959 : 43), although S. 

 antiqua and 5. multiseptata are not characterized by a differentiated radial ornamen- 

 tation like that found in the otherwise similar type species of his subgenus Viruella. 

 Those taken from the Meadowtown Beds and Spy Wood Grit are characterized by a 

 finely parvicostellate ornamentation only infrequently differentiated into sectors 

 delineated by thickened ribs, and by the rare development of rugae, acutely disposed 



