The proposal by Atkins (1938), that the Bivalvia can, on the basis of the composition of the latero-frontal ciliated tracts, be divided into two groups, the Macrociliobranchia and the Microciliobranchia, is examined. It is concluded that the Ostreidae, in which the latero-frontal tracts consist of compound cirri together with subsidiary simple cilia, should not be grouped with the Microciliobranchia. The remaining families in this group are characterized by the possession of latero-frontal tracts consisting of simple cilia only; the Pinnidae show some modifications of the latero-frontal tracts, the precise details of which are still to be determined. The form of the gill in the Anomiidae, Pectinidae, Limidae and Pinnidae is reviewed. It is suggested that in these families, possibly correlated with latero-frontal tracts consisting of simple cilia only, the collection and transport of particles by the gill for possible ingestion is primarily dependent upon the flow of water currents rather than direct ciliary action as in those bivalves which possess compound eu-latero-frontal cirri. The families possessing latero-frontal tracts consisting of simple cilia only are all included in the subclass Pteriomorphia, together with two families, the Mytilidae and Ostreidae, which possess compound latero-frontal cirri. Some workers already exclude the Mytilidae from the Pteriomorphia. It is suggested that before discounting the value of the laterofrontal tracts in indicating relations there should be a reappraisal of the position of the Ostreidae.