416 ELEMENTARY GEOLOGY 
. 
animals reach their culmination of development in this 
period, and have progressively declmed ever since. 
There is also a development of the cephalopod group 
Fig. 238. 
A group of Devonian cephalopods. 
(1, Gyroceras validum; 2, Orthoceras thoas; 3, Gomphoceras; 4, Cyrtoceras ; 
5, Cyrtoceras lineatus; 6, Cyrtoceras undulatum.) 
(Fig. 238), and an increase in the importance of reef- 
building corals. 
However, it is among the true fishes that the greatest 
progress is made. The Devonian has been called the 
