In Dipterus there are no marginal teeth, and the tail is heterocercal, not diphycercal, as in the other Dipnoans generally. Numerous species of Dipterus occur in Devonian rocks. In these the jugular plate is present, as in Uronemus. Dipterus valenciennesi is the best-known European species. Dipterus nelsoni and numerous other species are found in the Chemung and other groups of Devonian rocks in America.
Fig. 382.—Phaneropleuron andersoni Huxley; restored; Devonian. (After Dean.)
In the Ctenodontidæ the tail is diphycercal, and no jugular plates are present in the known specimens. In Ctenodus and Sagenodus there is no jugular plate and there are no marginal teeth. The numerous species of Ctenodus and Sagenodus belong chiefly to the Carboniferous age. Ctenodus wagneri is found in the Cleveland shale of the Ohio Devonian. Sagenodus occidentalis, one of the many American species, belongs to the coal-measures of Illinois.
As regards the succession of the Dipneusti, Dr. Dollo regards Dipterus as the most primitive, Scaumenacia, Uronemus, Ctenodus, Ceratodus, Protopterus, and Lepidosiren following in order. The last-named genus he thinks marks the terminus of the group, neither Ganoids nor Amphibians being derived from any Dipnoans.
Order Sirenoidei.—The living families of Dipneusti differ from these extinct types in having the cranial roof-bones reduced in number. There are no jugular plates and no marginal teeth in the jaws. The tail is diphycercal in all, ending in a long point, and the body is covered with cycloid scales. To these forms the name Sirenoidei was applied by Johannes Müller.
Family Ceratodontidæ.—The Ceratodontidæ have the teeth above and below developed as triangular plates, set obliquely each with several cusps on the outer margin. Nearly all the species, representing the genera Ceratodus, Gosfordia, and Conchopoma, are now extinct, the single genus Neoceratodus still existing in Australian rivers. Numerous fragments of Ceratodus are found in Mesozoic rocks in Europe, Colorado, and India, Ceratodus latissimus, figured by Agassiz in 1838, being the best-known species.
The abundance of the fossil teeth of Ceratodus renders the discovery of a living representative of the same type a matter of great interest.
Fig. 383.—Teeth of Ceratodus runcinatus Plieninger. Carboniferous. (After Zittel.)