EVOLUTION AND PALEONTOLOGY 255 



of Dipterus thick and enamelled but cycloid and overlapping. 

 In other Crossopterygians, the scales are cycloid and the re- 

 semblance is still greater as is evident from Fig. 23, A and B. 

 Holoptychius flemingi represented in A occurs in the Upper Old 

 Red Sandstone. 



There is good reason, as we have seen above, to believe 

 that the Old Red Sandstone is a fresh-water formation and that 

 these fishes, therefore, lived in fresh water. From the fact that 

 existing Crossopterygians and Dipnoi live in fresh water and 

 have respiratory air-bladders or lungs we may conclude that 

 this was the original function of the air-bladder, and that both 



Fig. 23. — A, Holoptychius flemingi from Upper Old Red Sandstone ; B, 

 Dipterus valenciennesi from Middle Old Red Sandstone. 



the air-bladder and the change in the dermal skeleton arose in 

 consequence of the change from a marine to a fresh-water 

 habitat. We cannot give a reason for a change in the dermal 

 skeleton, but in the case of the air-bladder there need be little 

 doubt that it arose in fishes originally of the Elasmobranch type 

 which had ascended from the sea into rivers or swamps where in 

 consequence of a warm climate and rotting vegetation there was 

 a deficiency of oxygen in the water and the fish acquired the 

 habit of taking air into the gullet. The difference between 

 the two types of these ancestral air-breathing fishes, the Cros- 

 sopterygian and the Dipnoan, was evidently chiefly due to the 

 difference in mode of feeding, the Dipnoan having tooth-plates 

 adapted to crushing and masticating hard or vegetable food, 



