x. 5~6 



MODERN LUNG-FISHES 



275 



different characteristics such as proportion of skull, nature of dermal 

 bones or dentition were divided into 3-8 grades. Each fossil genus 

 was thus given a score, the minimum being for characters appearing 

 in the oldest lung-fish, the hypothetical ancestor being o. Actual 

 scores ranged from 4 for the earliest to 100 for two of the living genera. 

 Plotting against time we see that there was an early acceleration of 

 evolution followed by very slow or zero change in the last 150 million 

 years (Fig. 164). It has also 



been shown that a similar 

 method applied to the evolu- 

 tion of coelacanths gives a 

 curve of similar shape (Schaef- 

 fer, 1952). 



6. Modern lung-fishes 



The three surviving genera 

 of lung-fishes (Fig. 163) are 

 mainly inhabitants of rivers 

 (though Protopterm lives in 

 large lakes) and they all 

 breathe air. Neoceratodas lives 

 only in the Burnett and Mary 



rivers in Queensland, the pools of which become very low and stagnant 

 in summer. Lepidosiren from the rivers of tropical South America, and 

 Protopterus from tropical Africa, can survive when the rivers dry up 

 completely. They dig into the mud, leaving a small opening for breath- 

 ing, and can remain in this state for at least six months. Remains of 

 cylindrical burrows found associated with dipnoan bones show that 

 this habit of aestivation has been adopted by the group at least since 

 Permian times. The three survivors all show similar deviations from 

 the conditions found in *Dipterus, but Neoceratodus has diverged less 

 than the other two. The tail fin is symmetrical (diphycercal) in all 

 three, with no trace of separate dorsal fins. The paired fins are of 

 'archipterygial' type in Neoceratodus, with an axis and two rows of 

 radials (Fig. 165). The scapula is covered by clavicles, cleithra, and 

 post-temporals, the latter articulating with the skull. The scales are 

 reduced to bony plates. 



The vertebrae are cartilaginous arches, the notochord remaining 

 as an unconstricted rod. In the skull there is also a great reduction of 

 ossification, the dorsal bones consisting of a few bony plates, forming 

 a pattern not obviously comparable with that of other forms (Fig. 



Fig. 165. Neoceratodus, showing the method of 

 walking on the bottom. A, forwards; B, back- 

 wards; C, resting. (From Ihle, after Dean.) 



