1 62 Evolution and Distribution of Fishes 



invaded the sea from early or Mid-Calciferous time on- 

 ward, while many still kept to inland waters. Of Crossop- 

 terygians, Rhizodus — in spite of formidable teeth — had 

 almost died out, but its allies Strepsodus and Rhizodopsis 

 were abundant in Europe, though rare or largely absent 

 in America. Coelacanthus, on the other hand, was abun- 

 dant in both countries. But the Actinopterygii, or "gan- 

 oids" in the older sense, were, as to number of individuals 

 and of species, the most abundant. The genus Rhadinich- 

 thys, that attained its climax in the Calciferous of Britain 

 and New Brunswick, persisted in several known species, 

 into the Coal Measures alike of Britain and of the United 

 States. But the genera Elonichthys, Platysomiis and Acro- 

 lepis are preeminent in distribution, as they occur from the 

 Calciferous up to the Permian, and from Bohemia on the 

 east to Illinois on the west. The last named genus also 

 seems to have extended from Russia to Cape Colony. 



The Physical and Biological Environment of 

 THE Permian. 



In Europe this formation is at times conformable 

 to and continuous with the Coal Measures, or may be more 

 or less unconformable. In North America it usually 

 grades upward so insensibly from the Coal Formation that 

 the term Permo-Carboniferous (or more accurately Carbo- 

 Permian as here used) has been widely applied. By gen- 

 eral consent palaeontologists have viewed it as bringing to 

 a close the oldest and most primitive of the three great 

 organic evolutionary stages of stratigraphic history, the 

 Palaeozoic. But all indications are that during deposition 

 of its strata, profound changes were proceeding in climatic 

 conditions; in the relative distribution of land and sea; in 

 biological stress and strain, or action and reaction amongst 

 organisms; in destruction of many previously existing 

 types; and in evolution of new and fast varying types; 

 in alternation of dry hot xerophytic with cool and even 

 glacial atmospheric states, over regions of the earth whose 



