614 GEOLOGY. 



berlin relative to the origin of the latter in fresh waters. 1 It will per- 

 haps yet appear that fresh water was the environment under which 

 some of the most radical of all the biological evolutions took place. 

 Another phase of association of the fresh-water life of the Car- 

 boniferous times is well preserved in a cannel coal deposit at Linton, 

 Ohio. 2 This coal is thought to have been formed from fine vegetable 

 debris borne into a fresh-water lake or lagoon, where it settled and 

 buried the relics of the local life, which consisted of "more than 20 

 species of fishes, and nearly 40 species of aquatic amphibians. New- 

 bury thought that their individual numbers mounted into the millions. 

 Among the fishes were crossopterygians (Ccelacanthus and Rhizodus), 

 actinopterygians (Eurylepis) , dipnoans (Ctenodus) and the peculiar 

 pleuracanthids (Orthacanthus, Diplodus, Compsacanthus). These las; 

 are regarded as elasmobranchs which then, as now, lived in fresh 

 water, though mainly marine. The amphibians belonged to the sub- 

 orders previously described. 



IV. The Marine Life. 



Because of the remarkably close adjustment of the sea-level to 

 broad areas of marshes and flat lands, two phases of sea life had suffi- 

 cient prevalence to be worthy of note. The first consisted of those 

 forms that habitually occupied the thin edges of the sea which, in 

 the form of estuaries, lagoons and shoals, crept in and out on the border 

 tract as the relations of land and sea oscillated, while the second 

 embraced the life of the more open seas. This distinction had doubt- 

 less always existed, but had not before reached equal importance, 

 and the first phase was not usually well preserved, whereas, in the 

 coal regions of this period, it constitutes the better part of the record. 

 In this phase, where sandy and muddy flats prevailed, the pelecypods 

 and gastropods, with certain of the fishes, predominated, while in 

 the more open seas the brachiopods, cephalopods and the clear-water 

 types more largely predominated. 



The progress of the fishes. — It is difficult to tell which of the fishes 

 should be regarded as marine, which as fresh water, and which as 

 common to salt and fresh water. It seems clear that much the larger 

 number of those found in the American Coal Measures lived in fresh 



1 Jour, of Geol., Vol. VIII, p. 400. 



2 Newberry, Mon. XVI, U. S. Geol. Surv., 1889, p. 211. 



