. Bulletin of the British Museum (Natural History) Geology Supplement. 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
. Bulletin of the British Museum (Natural History) Geology Supplement. 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 o
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