DEVONIAN SYSTEM AND AGE OF FISHES. 327 



with the age in which they culminate, or at least become a striking 

 feature. 



The Silurian, was, therefore, essentially an age of Invertebrates. In 

 number, size, and variety, these have scarcely been surpassed in any 

 subsequent period. The most characteristic orders were : Among 

 plants, Fucoids ; among animals, Cyathophylloid and Tabulate Corals, 

 Graptolites, Cystidean Crinoids, Square-shouldered Brachiopods, Beak- 

 less Gasteropods, Orthoceratites, and Trilobites. Orthoceratites and 

 Trilobites were the highest animals of the age, and the former were the 

 rulers and scavengers of these early seas. "We give below a table show- 

 ing, according to Barrande, the number of Silurian species described 

 up to 1872 : 



Sponges and other Protozoans . . . 153 



Corals 718 



Echinoderms 588 



Worms 185 



Trilobites 1,579 



Other Crustaceans „ 348 



Bryozoans 478 



Brachiopods 1,567 



Lamellibranchs 1,086 



Heteropods j ^qq 

 Pteropods ) 



Gasteropods 1,306 



Cephalopods. 1,622 



Fishes 40 



Which, with four of uncertain relations, make 10,074 species. 



Sectiok 2. — Devonian - System and Age of Fishes. 



The name Devonian was given to these rocks by Murchison and 

 SedgAvick, because in Devonshire the system occurs well developed, and 

 abounds in fossils. In England the system is usually unconformable 

 with the underlying Silurian, and sometimes with the overlying Car- 

 boniferous, as in Fig. 388. But in the Eastern United States, as 

 already stated, the Palaeozoics are conformable throughout (Fig. 264). 



Fig. 388.-5, Silurian; d, Devonian; c, Carboniferous (after Phillips). 



Area in United States. — The area over which the Devonian appears j 

 as a country rock in the Eastern United States is shown in map, page 291. 

 It borders generally the Silurian on the south and southwest, extending 

 with it far southward in the middle region, viz., in Indiana, Western 

 Ohio, and Kentucky. In the Basin Eange region, especially about 

 White Pine, Nevada, Devonian is known to exist, but the limits of 

 these areas are too imperfectly known to be described. 



Physical Geography. — In the eastern portion of the United States 

 the land of the Devonian age was approximately that of the Silurian 

 age already described, increased by the addition of the Silurian area, 



