Stratigraphical and paleoecological evidence indicates that lungfishes evolved in shallow marine conditions. Devonian genera had large gill chambers, and the details of bony supports of the gill arches of the Late Devonian Griphognathus whitei demonstrate that the arches were all functional. These data, together with an analysis of the body forms of the Devonian genera, indicate that they were dependent on gill (and possibly skin) respiration. The oldest known dipnoans, Uranolophus and Speonesydrion, are held to be representative to two lineages that can be recognized by their buccal and branchial features. One had a “rasping” dentition formed of denticles and marginal ridges that were continually shed and remodelled; the other had a “crushing” dentition characterized by the presence of variously modelled dentine masses that continued growth throughout the life of the animal. A list of buccal and branchial characters associated with these modes of feeding is presented. Because the relations of the Dipnoi have to be examined in terms of the features possessed by the group when it first appeared as a separate entity, the final part of the paper makes an attempt to define the primitive dipnoan morphotype. It is shown that many features taken to be diagnostic of the Dipnoi by some workers were not present in its early members; failure to recognize this fact has led to erroneous hypotheses about dipnoan-amphibian relations.