60 SHELVE DISTRICT 



Only a few other remarks can be made about the Whittery shells. As in other 

 Nicolella, muscle impressions are generally poorly defined. The ventral muscle 

 field was wider than long in young shells, but during growth there was an accelera- 

 tion in the anterior extension of the submedial diductor lobes so that the muscle 

 field of adult pedicle valves tended to be longer than wide ; and the mean percentage 

 width relative to length from the umbo (with variance) of 6 muscle impressions 

 between 3-0 and 5-5 mm long was 109-4% (425-84). 



Subfamily WHITTARDIINAE nov. 



Diagnosis. Piano- to concavo-convex orthids with a catacline to procline ventral 

 interarea, disharmonic costellate ornamentation and open delthyrium and noto- 

 thyrium ; ventral muscle field strongly bilobed with short parallel dental plates 

 and three or four pairs of short ridges radiating from the muscle field boundary ; 

 cardinalia consisting of a simple cardinal process and widely divergent brachiophores 

 ankylosed to the hinge line, adductor scars subtriangular, divided into inner and 

 outer pairs by vascula myaria ; ventral mantle canal system saccate, dorsal lem- 

 niscate to pinnate. 



Discussion. The new genus Whittardia represents an unexpected find within 

 such well-known Ordovician successions as those of Shropshire, because although it 

 is undoubtedly orthacean in its general organization and more particularly orthid 

 in the bilobed nature of the ventral muscle field and the simplicity of the cardinalia, 

 it is sufficiently unusual in a number of features to have aroused the attention of 

 palaeontologists had it been discovered earlier. Among such features, the dis- 

 harmonic ornamentation of the valves in the earlier stages of growth is the most 

 bizarre. Judged on the umbonal region of adult moulds, dorsal valves, up to 

 about 5 mm long, were convex and sulcate and ornamented by three symmetrically 

 disposed pairs of strong costae radiating from the umbo. The only other ornamenta- 

 tion developed within 2 mm of the umbo, in all valves examined, consisted of fine 

 closely-spaced lamellae. Thereafter at variable distances from the umbo inter- 

 calated costellae appeared, normally with a few thickened differentially. But with 

 the relative loss in strength of the primary costellae in valves longer than 5 mm, the 

 pattern became more evenly multicostellate with the costae arising by dichotomy or 

 intercalation and interrupted by fine concentric lamellae which were especially 

 conspicuous in the postero-lateral sectors free of ribs. The umbonal surface of the 

 pedicle valve, on the other hand, appears at first sight to have been quite different, 

 although details are usually obscured by the presence of a large mesothyridid fora- 

 men and the irregularity of the surface. The first-formed part of the preserved 

 external mould, for about 2 mm anterior of the foraminal edge, appears to be flat 

 and smooth and although valves up to about 5 mm long were sufficiently carinate to 

 accommodate the dorsal sulcus they were evenly and closely costellate and without 

 sign of accentuated interspaces corresponding to the strong costae of the brachial 

 valve. In the ventral interior, however, there were three or four pairs of low 

 narrow ridges radiating from the boundaries of the ventral muscle field and dying 



