FAUNA OF THE DEVONIAN PERIOD. 71 
circumstance, certainly very important, that of the 
Cephalopoda the Nautilus alone isfound. The Brachio- 
poda soon attain to their highest development, and have 
lingered on till now in a greatly reduced state. Among 
the Conchifera, the Dimyariz 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. 11, Palzeoniscus), 
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 
