62 SHELVE DISTRICT 



Genus WHITTARDIA nov. 



Name. In honour of the late Professor W. F. Whittard. 



Diagnosis. Subquadrate to semi-elliptical, sulcate shell with an irregularly 

 convex, carinate pedicle valve and a brachial valve initially convex but becoming 

 concave in adult stages of growth ; radial ornamentation of both valves costellate, 

 interrupted by fine lamellae, but with the finely lamellose umbonal region of the 

 brachial valve bearing only 6 symmetrically disposed accentuated costae becoming 

 subdued anteriorly ; ventral interarea gently curved catacline to procline, with 

 wide open delthyrium and mesothyridid foramen, dorsal interarea very short, 

 anacline, notothyrium open ; shell probably impunctate. 



Ventral interior with widely divergent tooth ridges and flat, oblique teeth supported 

 by short receding subparallel, widely spaced dental plates, postero-laterally enclosing 

 a bilobed muscle field with a relatively short undifferentiated adductor scar separat- 

 ing divergent submedial diductor lobes which encroach onto a pair of divergent 

 vascula media ; ventral mantle canal system probably saccate with vascula media 

 branching early ; 3 or 4 pairs of narrow, low ridges radiate from the boundary of the 

 muscle field and die away peripherally. 



Dorsal interior with simple cardinal process elevated above a shallow notothyrial 

 platform and wide short median ridge ; brachiophores short, widely divergent, 

 ridge-like, scarcely elevated above hinge-line and postero-laterally defining a pair 

 of elliptical sockets ; adductor scars subtriangular with posterior apices deeply 

 inserted in the notothyrial platform and divided into outer and inner sets by obliquely 

 disposed vascula my aria ; dorsal mantle canal pattern lemniscate or pinnate. 



Type species. Whittardia paradoxica sp. nov. from the Whittery Shales, 

 Shropshire. 



Whittardia paradoxica sp. nov. 

 (PL 9, figs. 7-15 ; PI. 10, figs. 1, 2, 4 ; Text-fig. 5) 



Diagnosis. Concavo-convex, subquadrate sulcate Whittardia with a pedicle 

 valve 34% as deep as long and a brachial valve 79% as long as wide, ornamented 

 by multicostellae with counts of 3 or 4 ribs per mm, 5 mm antero-medially of the 

 dorsal umbo ; ventral and dorsal muscle scars extending anteriorly for 24% and 

 46% of their respective valve lengths. 



Description. Variably subquadrate, sulcate Whittardia with a convex medially 

 carinate pedicle valve with a mean length relative to width of 66% (range 56% to 

 83%) and a mean depth relative to length of 34% (range 26% to 41%) for 4 valves, 

 and an irregularly concave brachial valve averaging 79% as long as wide (range 71% 

 to 84%) for 4 valves ; radial ornamentation becoming multicostellate by inter- 

 calation and dichotomy in adult shells with 3 and 4 ribs per mm, 5 mm antero- 

 medially of the umbones of 2 brachial valves in each case. 



Ventral interior with bilobed ventral muscle scar becoming longer than wide in 

 adult shells and extending forwards for an average of 24% of the length of 4 pedicle 

 valves (range 20% to 33%). 



