134 Evolution and Distribution of Fishes 



But the gradual evolution of Onchiis and Cladoselache, 

 of monacanthid and diplacanthid forms, that seem all 

 to be primitive selachians, from the late Silurian to the 

 close of the Old Red period, introduces a new and more 

 voracious type of fish. For while in Cladoselache and 

 Acanthodes, the teeth are of variable size, form, and adapt- 

 ability for grasping or tearing, in genera like Ischnacanthus 

 and Acanthodopsis they are strong and adapted for grasp- 

 ing prey. Their further striking evolution along increasing- 

 ly carnivorous lines will be traced, when we reach the next 

 great geologic period. 



In shortly reviewing now the Old Red Sandstone period, 

 we would regard this as the contemporaneous and fresh- 

 water geologic representative of the Devonian or marine 

 set of rocks, though in recent years the term "Devonian" 

 has been often applied to both. During deposition of these, 

 varying oscillations took place in the relation of the land, 

 of freshwater, and of marine areas. But throughout the 

 period a huge and practically continuous northern continent 

 — the North Atlantis of Freeh — was developing a rapidly 

 advancing plant and animal life. For to find Holoptych'nis 

 giganteus in Upper Old Red rocks of New York, Pennsyl- 

 vania Iowa, and Colorado, in the north of Scotland, in 

 Belgium and in West Russia; to find //. nohilissimus and 

 H. flemingi (Fig. 14) with a like Old World distribution; 

 to find closely related species like H. halli, and H. america- 

 niis along with H. giganteus in rocks ranging from New 

 York to Colorado, indicate a community of environal con- 

 ditions that must have been extensive and intercom- 

 municating. 



Fig. 14. Holoptychius flemingi. A crossopterygean fish from 

 Upper Old Red rocks of Scotland. About one-eighth natural size. 

 (From Traquair). 



