. Bulletin of the British Museum (Natural History) Zoology. Occasional exceptions can be found to these general rules. Thus, L. jamesi common in the Argentine and Guinea Basins does not occur to the north and to the east. Such a distribu- tion is known from other protobranch groups (Allen & Sanders in m/s, Sanders & Allen, 1965). In addition the benthic slope and abyssal bivalve faunas of the Argentine and Guiana Basins tend to have a high degree of endemism, but, unlike the impoverished fauna of Norwegian basin which also has a high degree of endemism, the isolated fauna off the East
. Bulletin of the British Museum (Natural History) Zoology. Occasional exceptions can be found to these general rules. Thus, L. jamesi common in the Argentine and Guinea Basins does not occur to the north and to the east. Such a distribu- tion is known from other protobranch groups (Allen & Sanders in m/s, Sanders & Allen, 1965). In addition the benthic slope and abyssal bivalve faunas of the Argentine and Guiana Basins tend to have a high degree of endemism, but, unlike the impoverished fauna of Norwegian basin which also has a high degree of endemism, the isolated fauna off the East coast of South America is relatively rich in species. This is possibly correlated in some way to the north moving, rela- tively organically rich bottom water. MORPHOLOGICAL EVOLUTION. Fig. 79 Ledella sp.: Left lateral external view of shell of the specimen from Station 123 and lateral internal view of right valve of specimen from Station 83 to show hinge detail. (Scale = 1 mm). depths is the L. pustulosa complex. This latter species appears to be in the process of speciation and here we recognise four closely related subspecies. Two, L. p. pustulosa and L. p. marshalli, while overlapping latitudinally in the North East Atlantic, are allopatric occurring on the slope abyssal rise and the upper abyssal depths respectively, the other subspecies, L. p. argentina and L. p. hampsoni, are geographically separate at abyssal rise depths. L. modesta, another common and widespread species, occurs somewhat deeper than the L. pustulosa complex. From these examples it is possible to make some general conclusions about the distribution of Ledella in the Atlantic and, from experience, also apply generally to the protobranch bivalves of the deep sea. 1) that common truely abyssal species are widespread in their occurrence. 2) common species of the slope and abyssal rise show sufficiently discernable population differences in shell form to an extent that true subspecies may be recognized. 3) rar
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