The Primitive Fishes 263 



olepis are distributed from W. Russia, across Scotland and 

 England into N. E. America. 



The Hyperplacoda (Anaspida) seem to be a greatly 

 less aberrant and less heavily armored group than all of 

 those already reviewed, except the first. The three genera 

 Birkenia (Fig. 41), Lasanius (Fig. 42), ^.nd Euphanerops, 

 occur from the Upper Silurian rocks of Scotland to the 

 Upper Devonian rocks of Canada. In not a few characters 

 they suggest a rather continuous advance from the Pro- 

 toplacoda to primitive members of the Microplacoda or 

 Elasmobranchii, that are referred to below. As is true 

 also of the Protoplacoda, the Microplacoda, and other of 

 the above great primitive groups, the Hyperplacoda de- 

 veloped no true bone-forming cells in any of their tissues. 



But at this stage we would now stop to inquire as to 

 the environal medium in which all of the above fishes 

 lived, and the possible means, as well as extent, of their 

 geographical distribution. As already claimed (p. no) 

 none of the rocks in which they are found, contain marine 

 organisms. In many cases land plants are spread out 

 alongside the fishes or in the same stratum of rock, while 

 not unfrequently phyllopod crustaceans, eurypterids, and 

 scorpions that are invariably of land or freshwater environ- 

 ment, are preserved alongside the fishes. Such conjointly 

 proclaim a freshwater life for all. No authentic case of a 

 marine environment has been observed or verified by the 

 writer, though from loosely recorded lists of organisms 

 taken from adjoining but quite distinct beds of the same 

 stratigraphic section, mistakes might be, and have been, 

 easily made. 



Regarding their distribution, lack of sufficiently exten- 

 sive evidence from the world's continents would suggest 

 caution as to conclusions. But to judge from present knowl- 

 edge we would accept it that fish-life probably evolved, in 

 early Silurian times, over some continental mass that 

 stretched from Russia westward to eastern North America. 

 Also that by late Silurian time several divergent, and in 

 not a few cases remarkably modified and cuirassed, lines 

 of fish-evolution with nearest affinity to very primitive 

 elasmobranchs (or Microplacoda) had appeared. Many 



