156 VERTEBRATE ZOOLOGY 



limb of Lepidosiren, the mud-dwelling habit, and the highly special- 

 ized air-bladder, used as a lung. 



The oldest representative of the Dipneusti is the genus Dipterus 

 which occurs along with the earliest known Crossopterygii and the 

 most primitive Actinopterygii in Devonian times. A comparison 

 between Dipterus and the early "lobe fins," such as Osteolepis, shows 

 obvious resemblances. Dipterus has acutely lobate paired fins, and 

 the skull bones are typically dipnoan in their lack of premaxillary, 

 maxillary, and dentary, and the presence of tritoral plate instead of 

 teeth. 



The larval stages of modem dipnoans are remarkably like those 

 of modern Amphibia. Those of Protopterus and Lepidosiren are 

 veritable tadpoles (Fig. 95, E and F) with external gills, tails with 

 unsupported median fins, blunt heads, and clinging habits. Just as in 

 amphibian larvse, they have a sucker or ventral cement organ for adhe- 

 sion. Evidently this tadpole larva represents an extremely ancient 

 developmental stage present in the common ancestors of these two 

 groups and retained with few modifications by the modem classes. 

 Indeed it seems certain that the whole early development of Amphi- 

 bia represents a more primitive evolutionary stage than does that of 

 any modern fish. It is customary to deal with amphibian embryology 

 as a step in advance of Amphioxus embryology and to derive the va- 

 rious types of fish development from some condition like that of the 

 amphibian. It is of some interest to know, therefore, that the dip- 

 noan fishes have a more primitive embryology (Fig. 96) than any 

 other living fishes, that the bony ganoids are next in primitiveness, 

 and that elasmobranchs, the more primitive ganoids, and the teleosts 

 show a specialized development, in some respects paralleUng that 

 seen in the reptiles and birds. 



Habits of Modern Dipneusti 



The existing genera of dipnoans are Neoceratodus of Australia, 

 Protopterus of Africa, and Lepidosiren of Paraguay. They are all 

 fishes of stagnant rivers and fresh-water pools. Neoceratodus is the 

 most primitive and shows less degeneration; Lepidosiren shows the 

 extreme reduction in fins and scales characteristic of the eel-hke type; 

 while Protopterus is just about intermediate between the other two. 

 Bridge has given good accounts of the lives of these singularly inter- 

 esting fishes. 



