652 GEOLOGY. 



an evolution of air-breathers, the key to it may lie in a more oxygenated 

 atmosphere, a point to which we shall return. 



The scant record of other land forms. — The Permian record of 

 the arthropods and of the terrestrial mollusks is very poor, and prob- 

 ably represents an impoverished state of these classes, but local excep- 

 tions will doubtless yet be discovered. 



777. The Fresh-water Life. 



The amphibians and some of the reptiles constituted, in a sense, 

 a portion of the fresh- water as well as the land life. Besides these, 

 fishes were abundant, locally at least. The beautiful little Paleoniscus 

 (Fig. 304) seems to have had its climax here, and almost disappeared 



Fig. 304.— A Permian fish, Paleoniscus macropomus, from Thuringia. (Restoration 

 by Traquair nearly one-half natural size.) 



at the close of the period. Fishes of similar structure, but with deep, 

 flat bodies, belonging to the Platysomus family, Fig. 305, were abundant. 

 The modern aspect of these fishes will be noted. With these were 

 some genera that had been prevalent before. Their associations show 

 that they lived in fresh waters, but they may also have lived in the 

 sea. 



There were fresh-water mollusks, some of which resembled unios. 

 The arthropod element was not well represented, though, so far as 

 known, it shows but little change from the Carboniferous. 



IV. The Marine Fauna. 



The withdrawal of the epicontinental seas from considerable por- 

 tions of the continents necessarily reduced the territory available 

 for shallow-water life, and introduced a restrictional evolution. There 

 was therefore at this time a coincidence between impoverishment on 



