154 VERTEBRATE ZOOLOGY 



SUB-CLASS III. DIPNEUSTI (Dipnoi). The Lung-Fishes 



This group of fishes has acquired an unusual interest because of the 

 belief, rather general until recent years, that from it the Amphibia 

 took their rise. A number of writers still appear to hold this view, 

 or at least to maintain that a close affinity exists between the Lung- 

 fishes and the Amphibia. Goodrich, for example, in the volume on 

 Cyclostomes and Fishes in Lankester's Treatise on Zoology, says:— 

 "The Dipnoi are among the most interesting of fish. On the one 

 hand, they have a close affinit to the Osteolepidoti (extinct 

 Crossopterygii) ; on the other, they present many striking points 

 of resemblance to the Amphibia, which cannot all be put down 

 to convergence." 



The group is distinguished by the following characters: — ^the paired 

 fins are rather slender and pointed; the scales are cycloid in form 

 and overlapping; the caudal fin is diphycercal; the upper jaw is firmly 

 fused with the base of the skull, making a holostylic skull; teeth are 

 largely lacking, and their place is taken by large tritoral dental plates, 

 supported by the palato-pterygoid and splenial bones; the premax- 

 illary and maxillary bones are absent, and the dentaries, usually 

 absent, are vestigial when present. It would appear from this sum- 

 mary of characters that the Dipneusti show more evidences of being 

 a degenerate and senescent group than one likely to give rise to a new 

 and successful class like the Amphibia. Most of the characteristics 

 of these fishes lead away from rather than toward amphibian condi- 

 tions. 



The question arises as to whether the characters mentioned were 

 more or less primitive in extinct than in modern lung-fishes. 

 The modern Dipneusti appear to have retained certain primi- 

 tive characters, such as the diphycercal tail fin and the entirely 

 cartilaginous condition of the primitive cranium. An examination 

 however of the earliest fossil Dipneusti, in which the tail is heterocer- 

 cal, the median fins broken up into isolated dorsals and ventrals, the 

 chondrocranium extensively ossified, and scales large and close to the 

 surface, show that the modern Dipneusti are degenerate and not 

 truly primitive. Additional degeneration is seen in the loss of maxil- 

 lary, premaxillary, and dentary bones, which are present in Amphibia. 

 The paired fins are also almost vestigial in some modern lung-fishes 

 and are so constructed as to make the derivation from them of any- 



