304 Evolution and Distribution of Fishes 



enclosed in these rocks. The amount of bituminous materi- 

 al may exceed io% of the entire rock material." But 

 further Geikie and his successors specially note that in the 

 Thurso beds "Estheria membranacea crowds the surfaces of 

 the shales and flagstones," and this feature equally extends 

 to strata of the Orkneys and other widely removed parts. 

 Also "plant remains abound in many parts of the Thurso 

 flagstone group. So far as yet known not a single true 

 marine plant occurs there, all the traces hitherto noticed 

 point to terrestrial vegetation," Flett then expresses his 

 own as well as Geikie's view when he says that all were 

 laid down in shallow tranquil water, and In landlocked 

 freshwater areas. So the above list applies not to crossop- 

 terygians only, but to a biological congeries which col- 

 lectively lived in Lake Orcadie at one time or another 

 during the Lower Old Red period. One gets an idea of 

 the relative abundance of Osteolepids at this time from 

 Geikie's statement that of 0. microle pi dolus as many as 

 a hundred individuals may be counted within a space of 

 three or four square feet, mingled with remains of Acan- 

 thodes and Cheiracanthiis. Though the material is usually 

 poor, a close parallel seems to have existed between the 

 above beds with their organisms and those of the Devonian 

 in Russia. So far as ascertained however physical and 

 organic connection with N. E. America had not yet or had 

 only recently been effected. Now none of these fishes has 

 been reported alongside a marine assemblage of organisms, 

 or in what seems to be a marine type of rock. Therefore for 

 the Crossopterygians as for others already studied, or to 

 be studied, all had a freshwater origin and environment, 

 up to the top of the Lower Old Red series and often showed 

 a great wealth of Individuals, 



Of the more evolved genera Holoptych'nis, Sauripterus, 

 Cricodiis, Eiisthenopteron ( Fig. 48), Glyptopomus, and 

 Onychodiis, these all appear in the Middle and Upper 

 Devonian of Scotland, Russia, or N. E. America, and often 

 abundantly. The diphycercal to usually heterocercal tail, 

 the relatively large strongly overlapping scales, the complex 

 tooth structure, the obtuse to acute paired fins, and the 



