414 THE SUCCESSION OF THE VERTEBRATE FAUNAS. 



In the yellow Old Red Sandstone of the Baltic province 

 of Russia, the Upper Devonian fish-fauna comprises compara- 

 tively large Arthrodira of the genera Homosteus and Hetero- 

 steus, which seem to have been toothless ; but these great 

 armoured fishes attain their maximum size and power just 

 before they become extinct, in a series of black shales (Waverly 

 Group and Cleveland Shales) forming passage beds between 

 the Devonian and Carboniferous systems in the United States, 

 especially in Ohio. Dinichthys is the commonest genus. Here 

 also occur the remarkable primitive sharks of the order Pleuro- 

 pterygii (Cladoselache), of which numerous well-preserved 

 skeletons are now known. 



CARBONIFEROUS AND LOWER PERMIAN. 



A new departure is now made in the history of the Verte- 

 brata. Certain impressions on slabs of sandstone from the 

 Upper Devonian of Pennsylvania, have been plausibly inter- 

 preted as the foot-prints of land-animals ; and quite at the 

 base of the Carboniferous system, at least in Britain, the 

 skeletons of such quadrupeds actually begin to occur. Through- 

 out the Carboniferous and Lower Permian formations of 

 Europe, the Stegocephalia, as these primitive land-dwellers 

 are termed, are represented by many genera and numerous 

 individuals; and since traces of a well-developed branchial 

 apparatus have been observed in several immature specimens, 

 the order to which the Stegocephalia belong, is usually assigned 

 to the lowest class of lung-breathers, namely, that of the 

 Batrachia. Some of these animals are crocodile-shaped, others 

 lizard-shaped, others limbless like snakes ; and it is remarkable 

 that closely similar, if not identical genera also occur in the 

 Upper Carboniferous of North America. The order is still met 

 with throughout the Trias of Europe, and it is also sparsely 

 represented in strata which may be either Permian or Triassic 

 in Spitzbergen, North America, South America, India, and New 

 South Wales. 



The Carboniferous and Lower Permian fish-fauna, preserved 

 in the same strata as these forerunners of the land-animals, is 



