CHAPTER XIX
DIPNEUSTI
Sub-Class III. Dipneusti (Dipnoi).
These singularly interesting Fishes are distinguished by their more or less acutely lobate paired fins and their overlapping cycloid scales, and by the fact that the bony dermal fin-rays of the median fins are much more numerous than their supporting radialia. Tail heterocercal or apparently diphycercal. Nostrils inferior. Vertebral column acentrous. The radialia of the median fins articulate with the contiguous neural or haemal spines and agree with them numerically. Skull autostylic. Premaxillae and maxillae absent, but a secondary lower jaw is represented by certain dermal bones of which tooth-bearing splenials are the most important, the dentary bones being absent altogether, or, if present, toothless and small. The cranial dermal bones include median as well as paired lateral plates, but their relations to those of other Fishes are very obscure. Two opercular bones are always present, but branchiostegal rays are unknown. One of the most important diagnostic features is the dentition. All Dipneusti agree in possessing large tritoral dental plates supported by the palato-pterygoid and splenial bones. The secondary pectoral girdle includes only cleithra and infraclavicles (clavicles). There is a pelvic girdle. Claspers absent. Of the four families of Dipneusti, two, the Ctenodontidae and the Uronemidae, are exclusively Palaeozoic. The third, the Ceratodontidae, is Mesozoic, and still survives. The fourth, the Lepidosirenidae, is known only by two existing genera.
Fig. 301.—Restoration of Dipterus valenciennesi. × ⅕. (From Traquair.)
Fig. 302.—Outline restoration of Phaneropleuron andersoni. Upper Devonian. (From Dollo, after Traquair.)
Fig. 303.—Dental plates of Dipterus valenciennesi, nat. size. A, Upper jaw; B, lower jaw. n, Position of the nostrils; p.p, palatine dental plates; p.pt, palato-pterygoid bones; sp, splenial teeth. (From Smith Woodward, after Traquair.)