FAUNA OF THE DEVONIAN PERIOD. 71 



circumstance, certainly very important, that of the 

 Cephalopoda the Nautilus alone is found. The Brachio- 

 poda soon attain to their highest development, and have 

 lingered on till now in a greatly reduced state. Among 

 the Conchifera, the Dimyariae 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 

 formation, 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 

 representatives 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. II, Palaeoniscus), 

 belongs only to the upper Coal and Zechstein formation ; 

 but it is necessary even now to point out the character- 

 istics of the true Ganoids which floundered about the 

 Silurian seas in somewhat extraordinary forms. Agassiz 



