In Carboniferous and Permian Epochs 151 



seem to have supplied the oil, the hard parts to have sup- 

 phed the contents of the probably true marine elasmobranch 

 "bone-beds." 



A detailed study of the numerous published lists of rock 

 sections given in the State surveys, or in some of the geo- 

 logical magazines, reveals how very unstable was the earth's 

 crust up to this time, and how frequently alternation of 

 freshwater and of marine conditions prevailed. The 

 natural outcome is that the geologist, who accurately takes 

 note of each horizon and of its fossil contents, constantly 

 records from adjoining and often thin beds, a freshwater 

 and then a marine list of organisms. An excellent example 

 of this is furnished by Condits "Conemau.gh Formation 

 in Ohio" (p9), which though dealing with the Higher 

 Carboniferous beds, is typical for those now treated of. 



As demonstrating also the widespread destruction to 

 fish-life that proceeded now, contemporaneously with the 

 evolution of new types, we may note in passing that the 

 unwieldy and giant fishes, like Coccostetis, Phlyctaenaspis, 

 Dinichthys, Titanichthys and Macropetalichthys, typical of 

 the North American Upper Old Red period, had wholly 

 died out by the close of the Mississippian, and had been 

 replaced by a higher, more lithe, and more voracious type 

 of fish, belonging to freshwater elasmobranch, and, to a 

 lesser extent, to crossopterygian affinities. 



The Mountain Limestone and Millstone Grit. Between 

 the Lower and Uppermost Carboniferous formations of 

 Europe, there are often intercalated two extensive sets of 

 rocks that in Britain have received the above designation. 

 The former is very largely of marine origin, though at 

 times showing intercalated freshwater beds. It may be 

 as much as 4,000 feet in thickness. The latter is singularly 

 devoid of organic remains, but varies from 1200 to 5500 

 feet in thickness. As the writer abundantly verified about 

 forty years ago, and as the published results of Kidston for 

 plants, and of Peach and Traquair for animals has equally 

 demonstrated since, the "Grit" beds separate two totally 

 distinct biological groups of organisms, that are specifical- 

 ly and often even generically or ordinally distinct. These 

 are the Lower Carboniferous, Culm or Mississippian, and 



