In Silurian and Devonian Epochs 



119 



In beds higher than these apparently, that Goodchild 

 calls the Orcadian, he and Flett have made known a rich 

 series of fishes that suggest a gradual splitting up, and 

 evolution of older types, into the two great elasmohranch 

 and ganoid groups of succeeding formations. Also while 

 some genera, like Coccosteiis (Fig. 9d), extend through 

 the entire series, others are typical of, or even extremely 

 abundant in, definite strata. The following is a partial 



Fig. 9. Fishes of the Old Red Sandstone (Devonian) period- 

 a Cephalaspis lyelli (after Lankester) ; b, Osteolepis microlepi- 

 dotiis ; c, Dipterus valenciennesii (both after Traquair) ; d, Coccos- 

 teiis decipiens; e, Acanthodes mitchelli (after B. N. Peach). All 

 reduced from A. Geikie. 



