i l l p o r , Isle o f Cumbrae, KA28 0 U.K. T he num ber of protobranch species of the continental shelves of the world comprise betw een 10 and 15 % of the total num ber o f bivalve species present. This is in contrast to the bivalve fauna of the deep sea which, distant from the lower continental slopes, is dom inated by the protobranchs. T he protobranchs may comprise more th an 70% of the bivalve species in a sample and more th an 95 % of the total num ber of bivalve specimens present. T he

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... ubclass Protobranchia has one o f the longest recorded geological histories and its continuing success, particularly in the deep sea, is probably due to a suite of physio logical characters th a t enable it to utilize a low and refractory food supply a t consider able depths and pressures. Probably as a result of the lack of com petition from bivalves o f more recent origin as well as the long stability of their environm ent, the deep sea bivalves show a radiation of form and habit th at is analogous to th a t shown by the more recently evolved lamellibranchs of the continental shelf. The study of the bivalve fauna of the deep sea helps in the understanding of the evolution and ecology of the Mollusca of late C am brian and early O rdovician periods. Pojeta (1971) , in his review of O rdovician Bivalvia, is of the opinion that, while it is reasonable to infer th a t there must have been C am brian representatives of the Class, all 17 genera described to th a t date are suspect for one reason or another. In this category Pojeta (1971) even includes the mid C am brian genus Lamellodonta recently and authoritatively described by Vogel (1962) . In m arked contrast, bivalves are well represented in the early Ordovician, and Pojeta (1971) believes th at by th at time six major lineages had been established. Furtherm ore, most O rdovician bivalves were infaunal suspension or deposit feeders, and m any of them were siphonate. O nly later, in the middle Ordovician, were epifaunal species common. T he m ajority of deep sea bivalves (table 1) belong to groups whose ancestry dates back to the O rdovician and the dom inant bivalve group of the deep sea, the Subclass Protobranchia are present in the earliest assemblages of the fossil record. They were well represented in the early O rdovician both in numbers and num ber of species (McAlester 1963 (McAlester , 1968 Pojeta 1971) . U ntil recently most O rdovician protobranchs with chevron-shaped taxodont teeth have been placed in a single family the Ctenodontidae. In none is there clear evidence of a resilifer, the recess or process where the internal ligament joins the shell. In most of these early protobranchs the hinge teeth are a continuous series on the hinge plate w ithout break below the umbo (figures 1 and 2); however, the teeth below the umbo are proportionately smaller th an those lateral to them. Pojeta (1971) notes th at the evolution of an internal ligament in protobranchs appears to have been a post-Ordovician development. This is not unreasonable for there are a priori grounds for believing th at the primitive condition is an external ligament possibly, though not conclusively, amphidetic in form (Allen i960; Yonge 1976). O ther notable points should be stressed. O rdovician protobranchs were a highly varied and successful g ro u p : both nuculoid and nuculanoid morphologies were present and existed at the same time. Both types are not only included within the family Ctenodontidae but are also