CHAPTEE 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 infra- 

 clavicles (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. 



Fam. 1. Ctenodontidae. — Body fusiform. Tail heterocercal 

 or apparently diphycercal. Excluding the anal fin, which is 



SOS 



