FAUNA OF THE DEVONIAN PERIOD. 71 



the Conchifera, the Dimyarise take the lead in the course 

 of the later period; and with regard to the Gasteropods, 

 we will merely observe that they constantly increase in 

 internal complexity and variety as they approach more 

 recent periods, and that the terrestrial and fresh-water 

 species are occasionally found in the carboniferous forma- 

 tion, though in number and variety they belong primarily 

 to the Tertiary era. To the Cephalopoda we must return 

 again. Of the Vertebrata in the Silurian strata we know 

 only the remains of peculiar Fishes whose kindred must 

 be sought among the sharks and rays. 



In the period of the Devonian or upper Transition 

 rocks, the surface of the earth had assumed, at least in 

 places, a more smiling appearance. Here begins the 

 first record of terrestrial plants. As to the character of 

 the fauna, the rapid decrease of the Trilobites is worthy 

 , of notice, and the appearance of the important genus of 

 the Cephalopoda, Clymenia, subsequently replaced by the 

 Ammonites. Above all, we must note the increased 

 abundance of fish which still form the sole representa- 

 tives of the Vertebrata, and held undisputed sway in the 

 seas of that period. Besides the sharks, there are the 

 mailed Ganoids. It is true, the fish, the hinder part of 

 which is here portrayed (Fig. 11, Palaeoniscus), belongs 

 only to the upper Coal and Zechstein formation; but it 

 is necessary even now to point out the characteristics 

 of the true Ganoids which floundered about the Silurian 

 seas in somewhat extraordinary forms. Agassiz terms 

 them Placoids, from the rhombic scales, provided with 

 a layer of enamel highly favourable to preservation, and 

 covering the whole surface in oblique rows.. The ver- 

 tebral column, as in the sharks, enters the upper flap of 



