(274) 



&£? ->'■#?£'? 



Fig. 163. The three living lung-fishes and their distribution. 

 A, Protopterus; b, Lepidosiren; c, Neoceratodns. (From Norman.) 



350 



250 



150 



Age / Millions of years) 



Fig. 164. Rate of evolution in lung-fishes. Each point represents the index of a single 

 genus obtained by taking twenty-six characters and rating them with grades of struc- 

 ture. The lowest value was given for the most primitive condition, the highest for the 

 most modern one. (After Westoll and Simpson.) 



