/2 THE DOCTRINE OF DESCENT. 



terms them Placoids, from the rhombic scales, provided 

 with a layer of enamel highly favourable to preserva- 

 tion, and covering the whole surface in oblique rows. 



The vertebral column, as in the sharks, enters the upper 

 flap of the tail and renders it strikingly unsymmetrical. 

 The Ganoids are, as comparative anatomy has proved 

 with certainty, a development of the shark-like fishes, if 

 not decidedly of a higher grade. The Ganoids, there- 

 fore, presuppose the shark. 



The carboniferous period owes its name to the enor- 

 mous accumulation occurring in its midst, of the 

 remains of terrestrial plants, fern-like Calamites, and 

 more especially of Sigillaria and Lepidodendra, stand- 

 ing between vascular Cryptogams and Conifers. They 

 formed tropical bog-forests, such as Franz Unger some 

 years ago attempted to restore in an ingenious compo- 

 sition. In these steaming primaeval forests, differing 

 from the early beginnings of antecedent periods by 

 their extent and luxuriance, new phases of animal 

 life become manifest scorpions, myriapods, and in- 

 sectsin other words, air-breathing Articulata, and 

 likewise the first air-breathing Vertebrata. The latter, 



