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The functional and adaptive morphology of six species of the genus Limopsis from deep water of the Atlantic is investigated and the following species and subspecies are described: L. aurita, L. surinamensis (new species), L. tenella, L. minuta, L. cristata cristata, L. cristata affinis, L. cristata intermedia (new subspeices), L. cristata lanceolata (new subspecies), L. galathea . Species of the genus Limopsis are among the relatively few suspension-feeding bivalves of the deep sea. The genus is not represented in shelf faunas. It is byssate, but the species described here show a change in habit from the exobyssate to the endobyssate condition and a corresponding reduction in the number and coarseness of the byssus threads produced. While the morphology of the species is similar in all the species described, differences being restricted to minor variation in the gills and palps, the viscera are confined to a small dorsal portion of the mantle cavity. Thus, while small body size is related to the paucity of food, production of a large thick shell may require a relatively small proportion of the energy input and yet confer advantage possibly in protection and in feeding mechanism. Eggs are relatively few and of a size indicating that the larvae are planktonic but non-feeding. The widely occurring species L. tenella shows differences in the shell characters of different populations. In L. cristata variation is such that it has been possible to describe subspecies.

Examination of the structure of species ofNucula,Leda,YoldiaandSolenomyaled Pelseneer (1891), in his classic work on the morphology of the Lamellibranchia, to establish for their inclusion the order Protobranchia. He regarded them as in almost all respects a primitive group, and Stempell (1898 a, 1898 b, 1899) came to very similar conclusions. Later (1898, 1911) Pelseneer extended his work on these animals, grouping all recent species in the three families Solenomyidae, Nuculidae and Ledidae ( = Nuculanidae). On the other hand, Neumayr (1891), on the basis of the hinge characters, grouped all but the first of these families with the Arcidae, Glycymeridae and Limopsidae in the order Taxodonta. Thiele (1934) elaborated this classification, dividing the Taxodonta into two divisions, Nuculacea and Arcacea, and subdividing the former into the four families Nuculidae, M alletiidae, Ledidae ( = Nuculanidae) and Solenomyidae. Finally, Douville (1912), in his illuminating sketch of a possible classification of the Lamellibranchia, regarded the Nuculidae and the Nuculanidae as affording some indication of the original characters of those Lamellibranchia which took to a more or less active life on the sea bottom, and the Solenomyidae as representing to some extent ancestral conditions to those which assumed life in burrows. Douville’s views have been well summarized by Davies (1933, I935).* Verrill and Bush (1897, 1898) have given the best account of the shell characters of the Protobranchia, while knowledge of m any aspects of their morphology has recently been extended by Heath (1937).