58 SHELVE DISTRICT 



Description. Piano- to slightly concavo-convex, subquadrate Nicolella with 

 cardinal angles almost invariably acute in all growth stages, pedicle valve evenly 

 convex 32% as deep as long and a brachial valve 64% as long as wide ; ornamenta- 

 tion consisting of 12 to 15 angular costae in 2, 1, 2 and 2 brachial valves more than 

 8*5 mm long with a mean wavelength (with variance) of i-og mm (0-008) at 5 mm 

 antero-medially of the umbones of 10 brachial valves, rarely branching in shells 

 longer than 7-5 mm with 3a usually arising first, concentric lamellae strong and 

 continuous but not occurring regularly ; ventral interarea short, curved orthocline ; 

 dorsal interarea very short, curved anacline. 



Ventral interior with small blunt teeth supported by widely divergent dental 

 plates extending anteriorly for 16% of the length of the pedicle valve, pedicle callist 

 small, ventral muscle field cordate, slightly wider than long in adult valves with the 

 submedial diductor lobes extending anteriorly for 34% °f the length of the pedicle 

 valve but not enclosing the shorter, undifferentiated median adductor scar. 



Dorsal interior with ridge-like erect cardinal process, chilidial plates present, 

 sockets slot-like defined by brachiophores extending laterally parallel to the hinge- 

 line for a distance equivalent to about one-third of the length of the brachial valve ; 

 dorsal adductor scars generally obscure but quadripartite with the larger anterior 

 pair extending forward of the dorsal umbo for 48% of the length of 3 valves (range 

 46% to 50%). 



Figured material. 



length width (mm) 



External and internal moulds of brachial valve (BB 35327a, b) 3-0 5-5 



External and internal moulds of brachial valve (BB 35328a, b) io-o 15-0 



External mould of brachial valve (BB 35329) 13-0 18-0 

 Incomplete internal mould of pedicle valve (BB 35330) - - 



Internal mould of pedicle valve (BB 35331) 12-0 15-0 



Horizon and localities. Whittery Shales : BB 35327 from Whittery Quarry 

 at south end of Whittery Wood near Chirbury (Grid Ref. SO 275981) ; BB 35328 

 from outcrops in west bank of the River Camlad 60 yds north-east of Marrington 

 Farm (Grid Ref. SO 272970) ; BB 35329, 35331 from outcrops half way down the 

 path going south from Marrington Farm to the valley bottom (Grid Ref. SO 272967) ; 

 BB 35330 from outcrops in stream at north end of Spring Coppice 865 yds south- 

 east of Hockleton Bridge (Grid Ref. SO 279997). 



Discussion. With the identification of the Whittery Nicolella as a form which is 

 indistinguishable from the Actonian N. actoniae (J. de C. Sowerby), the remarkable 

 morphological stability of that species becomes apparent, for it extends throughout 

 the Upper Caradocian and Lower Ashgillian without significant change in any of its 

 definable features (Williams 1963 : 352-356, Wright 1964 : 165-167). Tables 39-42 

 give the statistics for the main external and internal features of the Whittery speci- 

 mens, and their comparison with similar data for N. actoniae from the Acton Scott 

 Beds and the Portrane Limestone, and for N. actoniae obesa Williams from the 

 Gelli-grin Calcareous Ashes, shows no significant difference in any of the listed 

 characters. It is noteworthy that even in respect of the relative depth of the pedicle 



