ORDOVICIAN BRACHIOPODA 133 



the umbones of 1, 1, 7 and 4 brachial valves respectively ; impersistent postero- 

 lateral rugae very rare with a wavelength of 0-25 mm.. 



Teeth small, dental plates obsolescent, cordate ventral muscle field 71% as long 

 as wide and extending forward for 32% of the length of the pedicle valve ; adductor 

 scars small, impressed in 2 hollows in the umbonal secondary shell and divided by a 

 fine median ridge, splayed diductor scars divided by divergent vascula media. 



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

 widely divergent socket plates extending anteriorly for an average of 30% of their 

 lateral spread and for 10% of the length of 3 brachial valves ; notothyrial platform 

 poorly developed ; lophophore and muscle supports extending anteriorly for 73% 

 of the length of the brachial valve and consisting of a relatively short median septum 

 developing later than a pair of strong submedial septa normally with 2 additional 

 pairs of septa occurring lateral of the median triad, septa becoming united into a 

 cleft platform in 2 out of 10 specimens over 2-5 mm long. 



Type material. 



length width (mm) 



Holotype Internal mould of brachial valve (BB 35544) 2-8 5-3 



Paratypes External and internal moulds of pedicle valve 



(BB 35545a, b) 2-6 4-8 



Internal and incomplete external moulds of 



pedicle valve (BB 35546a, b) 3-0 6-0 



Internal and incomplete external moulds of 



brachial valve (BB 35547a, b) 3-5 6-5 



External and internal moulds of pedicle valve 



(BB 35548a, b) 5-3 9-5 



Incomplete internal mould of brachial valve 



(BB 35549) 1-5 



Horizon and localities. Spy Wood Grit : BB 35547 from exposures on top of 

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

 specimens from exposures 1100 yds NNE of Rorrington (Grid Ref. SJ 303015). 



Sowerbyella cf. sericea permixta Williams 



(PI. 24, figs. 2, 4-7, 10) 



1963 Sowerbyella sericea (J. de C. Sowerby) permixta Williams : 434. 



Diagnosis. Semicircular Sowerbyella with a brachial valve 50% as long as wide, 

 external postero-lateral surfaces infrequently ornamented by up to 7 pairs of im- 

 persistent rugae with a wavelength of 0-3 mm and by fine parvicostellae with modal 

 counts of 10 per mm, 2 mm antero-medially of the dorsal umbo and commonly 

 differentiated into sectors with a mean width of 0-67 mm ; ventral muscle scar 

 extending forward for 42% of the length of the pedicle valve ; dorsal interior with a 

 median septum occurring rarely and developing late, submedial septa strong, usually 

 flanked by a low platform with or without the differentiation of 2 pairs of lateral 

 septa and extending anteriorly for 66% of the length of the brachial valve. 



