The Primitive Fishes 285 



but a critical comparison of the associated plants, animals, 

 and strata, clearly demonstrates that the elasmobranchs 

 originated as, and in considerable degree remained, a 

 freshwater group throughout the above epochs. The fre- 

 quent entire absence of any fish remains from typical marine 

 strata that abound in brachiopods, pelecypods, cephalopods, 

 and marine crustaceans, is a strong argument against 

 the marine origin of fishes also. Their existence at first 

 in freshwater only of Silurian, also of early and mid 

 Devonian; their gradual spread seaward in anadromous 

 manner during Calciferous or Mississippian time; and their 

 rapid evolutionary expansion, in the Carboniferous Lime- 

 stone and earlier Coal Measures periods, into the sea, 

 constitute a sequence of events. 



Numerous examples of Acanthodidae, Pleuracanthidae, 

 Cladodontidae, Petalodontidae, Cochlidontidae, Cestra- 

 ciontldae, and Ptychodontidae are so inextricably connected 

 with undoubted freshwater forms, that the writer is com- 

 pelled to accept the freshwater origin of the Elasmo- 

 branchii as a whole. But during late Devonian and Calci- 

 ferous or Mississippian time some genera reached the sea, 

 and as in nearly all such cases, for plants as well as animals 

 (7:324-410), they there multiplied abundantly and evolv- 

 ed numerous new types. The climax of evolution for the 

 phylum, in palaeozoic time, was reached from the Carboni- 

 ferous Limestone to near the close of the Coal Measures. 



Combining the records of Woodward's Catalogue 

 (v. 1, 2) with recent additions, and reckoning on a conserv- 

 ative basis, it appears that there were at least fifty-five 

 genera that included about three hundred and sixty species. 

 Of these about twenty genera, and close on one hundred 

 species remained as freshwater or as anadromous species, 

 the remainder became largely or wholly mairine. The 

 distribution of these over the world follows very closely 

 that already cited for other groups. But in America the 

 westward extension of the freshwater ones was carried to 

 Illinois, Iowa, Missouri, and in a few cases even, if the 

 records and localities are alike correct — to the coal strata 

 of Nebraska and Kansas. 



