ORDOVICIAN BRACHIOPODA 



61 



out peripherally, which could well have been complementary to the accentuated 

 costae of the brachial valve. This arrangement suggests that the valves were 

 imperfectly matched along the commissure, and such a loose fit is compatible with 

 the weak articulation indicated by the simplicity of the teeth, the shallowness of the 

 sockets, and the powerful development of the adductor musculature as inferred from 

 the wide ventral impression and the deep posterior insertion of the dorsal attachment 

 areas. 



The other unusual features of the new genus are the catacline to procline ventral 

 interarea and the profile of the brachial valve. The disposition of the ventral 

 interarea was not unique among orthaceans but it was associated with a large 

 mesothyridid foramen which must have accommodated a thick pedicle that was so 

 short as to bring the pedicle valve into contact with the substratum ; the irregular 

 contours of the valve are not post mortem features but expressions of growth in a 

 physically restrictive environment. The longitudinal profile of the brachial valve 

 was initially convex, but beyond the 2 mm growth stage the valve became gently 

 concave. This attitude, however, was accentuated or reversed along a few narrow 

 arcs resembling concentric rugae or, if incomplete, incipient geniculation. Such 

 bands corresponded to only vague concentric indentations in the ventral interior, 

 again indicative of the degree to which the valves grew independently of each other 

 at the edges. 



No other described orthacean compares closely with Whittardia : indeed the 

 principal difficulty is deciding the suprageneric group to which it is best assigned. 

 There is a superficial resemblance to the dinorthid Plesiomys (Retrosistria) in the 

 ventral muscle impression but this is due to the catacline attitude of the ventral 

 interarea in both stocks. Yet the simplicity of the cardinalia, and the bilobed nature 

 of the ventral muscle scar and the mantle canal systems, suggest that the ancestor 

 of Whittardia was more likely to have been an orthid and, provisionally at least, a 

 monotypic subfamily of the Orthidae has been created to accommodate the new genus. 



tooth 



cardinal process 



socket 



brachiophore 

 dental plate 



muscle scar 

 mantle cana 



Fig. 5. Diagrammatic views of (A) the ventral and (B) the dorsal interiors of 



Whittardia. 



