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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).