134 SHELVE DISTRICT 



Description. Semicircular Sowerbyella with acute cardinal angles becoming 

 roundly obtuse in late growth stages, planar to gently concave brachial valve 50% 

 as long as wide, pedicle valve 19% as deep as long, evenly convex to subcarinate in 

 transverse profile ; ventral and dorsal interareas apsacline and anacline respectively 

 with a supra-apical foramen and a small pseudodeltidium and complementary 

 chilidium ; radial ornamentation normally unequally parvicostellate (in 25 out of 

 32 brachial valves) with rib counts of 8, 9, 10, 11 and 12 per mm, 2 mm antero- 

 medially of the umbones of 2, 7, 15, 7 and 1 brachial valves respectively, with a mean 

 width of sectors delineated by thickened costellae of 0-67 mm ; up to 7 pairs of rugae 

 with a wavelength of 0-3 mm usually poorly and impersistently developed in the 

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



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

 as wide and extending anteriorly for 42% of the length of the pedicle valve ; adductor 

 scars small, 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 33% of their lateral spread 

 and for 13% of the length of the brachial valve ; notothyrial platform poorly devel- 

 oped ; lophophore and muscle supports normally consisting of 3 pairs of septa with 

 a strong submedial pair extending anteriorly for 66% of the length of the brachial 

 valve making up a low cleft platform ; a short median septum was developed in only 

 3 out of 20 valves less than 4 mm long and in 9 out of 24 valves more than 4 mm long. 



Figured material. 



length width (mm) 



External and internal moulds of pedicle valve (BB 35551a, b) 7-0 13-0 

 External and internal moulds of brachial valve (BB 35552a, b) 3-8 7-5 



External and internal moulds of brachial valve (BB 35553a, b) 4-2 8-2 



External and internal moulds of brachial valve (BB 35554a, b) 47 9-0 



Horizon and locality. Aldress Shales exposed in the bank of Ox Wood Dingle 

 at the south-west corner of Ox Wood a few yards north of the Rorrington-Wotherton 

 road (Grid Ref. SJ 290007). 



Sowerbyella cf. sericea (J. de C. Sowerby) 

 (PL 24, figs. 11, 13, 14, 16) 



1839 Leptaena sericea J. de C. Sowerby : 636. 



1928 Sowerbyella sericea (J. de C. Sowerby) Jones : 414. 



1963 Sowerbyella sericea (J. de C. Sowerby) Williams : 430. 



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

 and a pedicle valve 26% as deep as long, ornamented by up to 9 pairs of impersistent 

 rugae with a wavelength of 0*3 mm, and unequally developed parvicostellae with a 

 modal count of 9 per mm, 2 mm antero-medially of the dorsal umbo ; ventral muscle 

 scar extending forward for 30% of the length of the pedicle valve ; dorsal interior 



