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Summary. 1 The internal structure and the ciliary sorting mechanisms of the stomach are described for representatives of twelve families of Filibranchia and Pseudolamellibranchia. 2 The variable structural features of the stomach are examined individually, and it is shown that only one of these is suitable for use as a diagnostic in assigning the stomachs of these lamellibranchs to a limited number of types. This feature is the form of the major typhlosole. 3 The major typhlosole may sweep smoothly from the orifice of the mid-gut to that of the left pouch, accompanied on its course by the ciliated rejection tract of the intestinal groove. Alternatively a long slender tongue may pass forwards from the major typhlosole and penetrate deeply into the food-sorting caecum, this tongue being accompanied by a similar extension of the intestinal groove. No intermediate condition was found, and on this basis the stomachs in eight families were assigned to Type III, in which the major typhlosole possesses such a tongue. These families are the Arcidae, Glycimeridae, Limopsidae, Pinnidae, Vulsellidae, Pteriidae, Mytilidae and Ostreidae. 4 It is considered that the evolution of a tongue of the major typhlosole, extending into the food-sorting caecum, could not have occurred independently on two or more occasions, nor is it likely that such a tongue, once having been formed, could have been lost independently on two or more occasions. It is concluded, therefore, that the above mentioned eight families constitute the end products of one phylogeny of lamellibranchs. 5 In the remaining four families, the Trigoniidae, Limidae, Pectinidae and Anomiidae, the major typhlosole lacks a tongue. The stomachs in these four families are assigned to Type IV, and attention is drawn to the fact that in many Eulamellibranch families the internal structure of the stomach is similar. 6 It is suggested that evolution of the structure and function of the interior of the stomach has played an important part in the adaptive radiation of the Lamellibranchia. 7 It is reiterated that the Protobranchia constitute a phylogeny which is distinct from all other lamellibranchia and that it is undesirable to link the Protobranchia with the Arcacea to form the order Taxodonta.

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).