The Structure of Certain Palaeozoic Dipnoi.

This paper includes the first complete accounts of the circumorbital region and opercular apparatus of Dipterus valenciennesi, and of the lower jaw of D. platycephalus. It gives very nearly complete descriptions of the skull, lower jaw, and clavicular apparatus of Sagenodus and Ctenodus which much extend our knowledge of these fish. In it the structures of the anterior end of the rare fish Uronemus and Conchopoma are described and figured for the first time. Dipterus is shown to be directly comparable with Osteolepids in the structure of the opercular apparatus and the lower jaw, in addition to the many previously known resemblances. It is thereby shown that, as its early date would indicate, it is the most primitive known Dipnoan. Ctenodus and Sayenodus prove to be closely allied, and a detailed comparison shows so great a similarity between the latter fish and Ceratodus as to leave no doubt that it is essentially ancestral to it. Uronemus and Conchopoma resemble one another only in the reduction in them of the tooth-plates to isolated denticles. In the structure of the palate and of the roof of the head they differ so much that they must represent widely-separated stocks. In the main, the trends of Dipnoan development suggested by Watson and Day are confirmed. It is, however, pointed out that the structure of the neural cranium of the Osteolepids, as described by Bryant in Eusthenopteron, is such that the Dipnoi cannot be direct descendants of that group, but that with it and the Amphibia they arose together from common ancestors at a time before the Middle Devonian.