GEA TER o 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.—Pody fusiform. Tail heterocercal 
or apparently diphycercal. Excluding the anal fin, which 1s 
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