152 Evolution and Distribution of Fishes 



the Upper Carboniferous, True Coal Measures, or Penn- 

 sylvanian. 



Probably no other formation has been so fully and 

 readily accepted as of freshwater or terrestrial formation 

 than the latter, for it and the important coal-beds of the 

 world have been inseparably linked in human thought for 

 the past fifteen decades. So while we would view the 

 petroleum oils of the Old Red and the Lower Carbonifer- 

 ous formations as being fish derivatives, it is equally true 

 that the pure carbon or coal of the present group is of 

 vegetable origin. But from its intimate relation to an 

 abundant fish-life, it is not at all improbable that some types 

 of coal, like the cannel or parrot, largely owe their bitumin- 

 ous properties to intrinsic permeations of fish oil.* 



But before treating of the biological assemblage that 

 characterizes the Coal Measures, it should be observed that 

 the invasion of marine regions by elasmobranch fishes main- 

 ly, which started in the Calciferous period or possibly 

 earlier, became a most pronounced event during deposition 

 of the Mountain Limestone, and was continued into the 

 Coal Measures. This migrational change is fully discussed 

 in Chapter 9 (pp. 275-79) ^^^ so consideration of the fish- 

 groups involved can be deferred meanwhile. 



The Coal Measures {or Pennsylvania beds.) The high 

 economic value of the rocks composing this division has 

 caused detailed study of its physical and biological features 

 alike. But it should be borne in mind that though the great 

 mass of strata composing it suggests a freshwater and land • 

 origin, continued though less prolonged oscillation of the 

 earth's crust than in preceding epochs, often caused tempo- 

 rary marine invasion, over large areas that fundamentally 

 remained as land masses. Probably the most careful and 

 extensive demonstrations of this are brought forward by 

 Kirkby in his paper "On the Occurrence of marine fossils 

 in the Coal Measures of Fife" (700:378); also by J. 

 Ward (707:42), and by J. T. Stobbs (702:495). The 

 last named points out how easy it is to confound fossils 



* This subject is fully considered in the writer's volume entitled "Fishes the Source 

 of Petroleum"' (1923). 



