286 HISTORICAL GEOLOGY. 



allied to the horseshoe crab (Limulus) than anything else. 

 They are so abundant, so well preserved, and so easily rec- 

 ognized, even by the untrained eye, that they are a very 

 valuable means of identifying Paleozoic, and especially 

 Silurian, strata. 



Anticipations of the Next Age. The most highly 

 organized and most powerful animals of Silurian times 

 were undoubtedly the Orthoceratites and the Trilobites. 

 The Orthoceratites especially were the tyrants and scaven- 

 gers of those early seas ; yet, in the uppermost Silurian 

 are found a few insects, scorpions, and cockroaches, and a 

 few fishes similar to forms far more abundant in Devonian. 

 It is better, therefore, to regard these as anticipations. 



SECTION III. DEVONIAN SYSTEM. THE AGE OF FISHES. 



Kock-System ; Name. The name Devonian was given 

 to these rocks because first studied with success in Devon- 

 shire. In Scotland they were called Old Eed Sandstone, 

 by Hugh Miller. In England it is often unconformable 

 on the Silurian, but in the Eastern United States, as al- 

 ready stated, the Paleozoics are conformable throughout. 

 Nevertheless, even in America there is a great change of 

 life-forms at this point of time ; and, moreover, the first 

 introduction here of a new reigning class viz., fishes, 

 and a new great department of animals viz., Vertebrata, 

 or backboned animals, is a prodigious advance, and en- 

 titles this to be considered a distinct age. It is well to 

 note, however, that some anticipations of this great ad- 

 vance are found in the Silurian. 



Area in the United States. By examining the map 

 on page 272, it will be seen that in general the Devonian 

 rocks border on the Silurian area on the south and west 

 and extend far south in the middle region. In the Rocky 

 Mountain region there are considerable areas of Devonian, 

 but their limits are too little known to be described. 



