u8 SHELVE DISTRICT 



length width (mm) 

 Paratypes External and internal moulds of brachial valve 



(BB 35481a, b) n-5 



External and incomplete internal moulds of 

 pedicle valve (BB 35482a, b) 12-0 16-5 



Internal mould of brachial valve (BB 35483) 14-5 22-5 



Horizon and localities. Whittery Shales : BB 35480 from Tuffaceous Shale 

 exposed in the lower part of the western bank of the River Camlad 60 yds north-east 

 of Marrington Farm (Grid Ref. SO 272970) ; other specimens from Whittery 

 Quarry at the south end of Whittery Wood near Chirbury (Grid Ref. SO 275981). 



Bicuspina modesta sp. nov. 



(PI. 20, figs. 3-8) 



Diagnosis. Subquadrate Bicuspina with a dorsal fold about half as wide as the 

 valve length, commonly bearing a median costa which usually branches within 5 mm 

 of umbo, lamellose with 5 lamellae developed between 5 and 6 mm antero-medially. 



Description. Small, biconvex, subquadrate Bicuspina with obtuse or, more 

 rarely, orthogonal cardinal angles ; pedicle valve 24% as deep as long with a flat- 

 bottomed median sulcus bounded by parallel sides and evenly convex flanks, brachial 

 valve 75% as l° n g as wide and 35% as deep as long with a splayed median fold 50% 

 as wide as the valve is long ; ventral interarea curved apsaline, pseudodeltidium 

 with narrow median fold, dorsal interarea vestigial ; radial ornamentation of 

 rounded ribs with a mean wavelength of 0-5 mm (variance 0-08), 5 mm anterior of 

 the umbones of 27 brachial valves, median costa (flanked by 2 laterals) occurs on 

 folds of 34 out of 37 brachial valves and bifurcates within 5 mm of the umbones of 

 19 valves ; 11 to 17 ribs occur on the flanks of brachial valves between 5 and 7-5 mm 

 long ; concentric ornamentation of overlapping lamellae with 4 or 5 occurring 

 between 5 and 6 mm anteriorly of the umbones of 2 brachial valves in each case. 



Pedicle tube and passage averaging 11% of the length of the pedicle valve ; small 

 teeth supported by acutely divergent dental plates extending forward for 68% of 

 their anterior separation and 15% as long as the valve ; muscle field poorly impressed 

 extending forward for 55% of the length of the pedicle valve. 



Cardinalia consisting of forked posteriorly curving cardinal process and short, 

 acutely divergent, pointed brachiophores ; sockets disposed parallel to the hinge- 

 line extending laterally for 26% of the length of the valve, usually defined distally 

 by ridges of secondary shell. 



Type material. 



length width (mm) 

 Holotype External and internal moulds of pedicle valve 



(BB 35510a, b) io-o 12-0 



Paratypes External and internal moulds of brachial valve 



(BB 35511a, b) 7-0 8-5 



External and internal moulds of brachial valve 



(BB 35512a, b) 9-5 n-5 



