498 Evolution and Distribution of Fishes 



Though we would not as yet attach much importance to 

 the fact, it should however be kept in view that the area 

 over which conodonts (p. 107) are distributed agrees ex- 

 actly with that above given as the centre for evolution of 

 fishes as a great group. 



It is not the writer's purpose to attempt a complete — 

 even superficial — review of the geographic and geologic 

 indications yielded by the great and persistent group of 

 the Elasmobranchii, except in the two ancient divisions, 

 Pleuracanthidae and Acanthodii. For we cannot as yet deal 

 fully and intelligently with most of them. 



The seven genera of the Acanthodii are known from 

 localities that are identical with or closely related to those 

 for Arthrodira and early Dipneusteans, except that the 

 area of occupation is even more limited, as regards the 

 earlier species of the group. For most have been met with 

 in the Lower Old Red only over the British, and specially 

 over the north-east Scottish region, where their richness in 

 individuals is at times remarkable. In the Upper Devonian, 

 and onward to the Permian where they die out, a steady 

 geographic extension of genera like Acanthodes and Acan- 

 thodopsis took place along the lines of migration already 

 demonstrated. We therefore find species of Acanthodes in 

 the Upper Devonian of Scaumenac in Canada, in the Calcif- 

 erous formation of S. Scotland, in the Coal Measures of 

 England and Scotland, also in the Permian of France, 

 Central Germany and Bohemia. Not unfrequently there- 

 fore they are companion-forms with dipnoans, arthrodires, 

 and in the Permo-Carboniferous with crossopterygians. 

 But so far as at present known they never transgressed the 

 limits of the N. Atlantis continent. 



The Pleuracanthidae and Cladodontidae, that make up 

 the Order Ichthyotomi, conform in distribution to the last, 

 except that they are somewhat later in geologic appearance. 

 The single genus Pleuracanthus (including Diplodus) is 

 first met with in the Lower Carboniferous of the east 

 Scottish area and of central England, but extends its range 

 into the Upper Coal not only of that region; it occurs in 

 practically all the coal fields of Britain; at Commentry in 

 France; also abundantly westward to the Ohio, Indiana, 



