236 Evolution and Distribution of Fishes 



all of which are allied to genera still found in the fresh- 

 waters of South America. Von Ihering observes that these 

 schists crop out and are worked at several localities for 

 the crude petroleum oil. From the oil, supplies of gas, 

 paraffin, petroleum and sulphuric acid are obtained. 



But in recent years, since many of the Eocene beds have 

 been examined from the economic standpoint, richly bitu- 

 minous rocks that seemingly owe their oils to decomposition 

 of marine fishes are now known. Thus in Texas and ad- 

 jacent states the rich oil beds that have been actively ex- 

 ploited during the past fifteen years, are also those which 

 contain an abundant and varied marine fish fauna that is 

 described in detail in another work. These belong to the 

 Cretaceous system, and seem in their faunal and strati- 

 graphic character to resemble other bituminous beds In 

 several parts of the world. Again the extensive Oligocene 

 and Miocene oil strata of western California owe their 

 oil-content, according to the present writers views, to the 

 marine fish fauna that has been only imperfectly described 

 as yet, but which Includes genera that belong to elasmo- 

 branch and teleostean families.* 



The giant amphibians and reptiles of the two previous 

 epochs, had now almost disappeared, though smaller sala- 

 mandroid forms were left. Turtles, crocodiles, and snakes 

 were fairly common, as their entire skeletons or smaller 

 remains, testify. But the most noteworthy feature was 

 the appearance of representatives of all the great divisions 

 of the mammals, with the exception of the two highest. 

 The writer has elsewhere suggested (/ : 496) that these all 

 originated by modification and increase In size of various 

 more primitive marsupial ancestors. As with the giant 

 amphibians of the Permian, the giant reptiles of the Juras- 

 sic and Cretaceous, so with the giant mammals of the pres- 

 ent epoch; all of these furnish a rough and approximate 

 measure of the great amount of evolutionary advance that 

 some organisms may undergo, alongside others that vary 

 slowly. 



*For detailed description of these see the author's volume, "Fishes the 

 Source of Petroleum." 



