482 GEOLOGY. 



by hypothesis, laid down in more or less local basins, no exact correla- 

 tion of them is possible at present and they may best be considered 

 together. The general faunal conception is therefore that in the 

 Appalachian tract and in the Canadian provinces lying to the north- 

 east of it, as well as in Great Britain and Russia, there were many 

 lodgment basins that were progressively filled by land-wash and fresh- 

 water sediments, and that these basins were the home of a fresh- water 

 or brackish-water fauna, in which ostracoderms and fishes were the 

 leading elements, and crustaceans their chief colleagues. 



The remarkable deployment of the crustacean-ostracoderm-verte- 

 brate group. — No more suggestive combination of ancient life is pre- 

 sented by the geological record than that which is found in these 

 supposed fresh- water deposits. The type was foreshadowed by the 

 eurypterids and fishes, or fish-like forms, that appeared in the closing 

 stages of the Silurian, but the record of that time is too imperfect 

 to disclose its deeper significance. Even with the much superior 

 material of the Devonian period, the more profound significance is 

 only just beginning to be realized. The center of interest is a unique 

 group of ostracoderms, which were at first interpreted as placoderm 



Fig. 213. — Restoration of Cephalaspis seen from the side. (After Patten.) 



fishes, and later classed with the jawless fishes (Agnatha, lampreys, 

 etc.), but which seem now to be clearly proven * to be an entirely dis- 

 tinct class lying between the arthropods and the vertebrates, and 

 having some of the characteristics of each, but not truly belonging 

 to either. Their supreme interest lies in the force they give to the 

 suggestion that the vertebrates sprang from the arthropods. 



1 W. Patten, On the Origin of Vertebrates, with special reference to the structure 

 of the Ostracoderms. International Zool. Cong., Berlin, 1901. On the Appen- 

 pages of Tremataspis, Am. Nat., XXXVII, 1903, p. 223. On the Structure of the 

 Pteraspidce and Cephalaspidw, Am. Nat., XXXVII, 1903, p. 827. New Facts con- 

 cerning Bothriolepis, Biol. Bull. No. 2, 1904, p. 113. 



