Distribution of Primitive Fishes 497 



to Upper Silurian times, and representatives have been 

 reported from much the same localities as above cited for 

 the Arthrodira. 



In structure, habits, and capacity of response to en- 

 vironal agents, all of the genera that make up the above 

 three groups closely resemble the Arthrodira, and so after 

 invading nearly the same territory, and living at times side 

 by side with the latter, they disappeared, before the dawn 

 of the Carboniferous period. In their earliest evolution 

 they antedated the Arthrodira, but resembled them in that 

 the oldest and most primitive genera Auchenaspts, Euker- 

 aspis, Cephalaspis, Cyathaspis and Scaphaspis probably 

 originated in the freshwaters of a Silurian tract of land, 

 that may have fairly corresponded with the North Atlantis. 



The isolation of this northern continent from a con- 

 tinuous and massive S. Atlantis-Gondwana continent, up to 

 nearly the close of the Devonian; and the gradual formation 

 later of a fairly wide connection between N. Atlantis and 

 S. Atlantis-Gondwana, as compared with Freeh's outlines, 

 would serve to explain satisfactorily the migrations of 

 several fish-groups from a northern and ancestral home, 

 to African, Indian, and Australian areas of ultimate in- 

 vasion. 



The small group of the Coelolepidae consists of the 

 genera Coelolepis, Thelodus, Lanarkia, and Ateleaspis that 

 are distributed from Scotland to W. Russia; and like some 

 groups studied above, very likely evolved in some part of 

 this territory, also amid freshwater surroundings. It may 

 here be added that, apart from any question as to the taxo- 

 nomic affinities of Palaeospondyhis, that remarkable fish 

 came from the same territory as the preceding. 



All five of these genera we would regard as the most 

 primitive of known fishes. So this would cause us to accept 

 at least as a temporary conclusion that fish-life originated 

 in freshwaters over a limited area of land that extended 

 from W. Russia to Scotland. It should be borne in mind 

 however that this land and its enclosed fishes may have 

 extended further west and even north, but geologic changes 

 of far-reaching nature have destroyed the evidence, if such 

 actually existed. 



