300 



PALAEOZOIC SYSTEM OF ROCKS. 



est to the geologist because it marks the outline of the earliest Silurian 

 sea, and contains the remains of the earlest Silurian fauna. Indeed, 

 we may say it contains the remains of the earliest known fauna. It is 

 true, the lowest Ehizopods probably existed in Archaean times, but 

 these can not be said to constitute a fauna. With the very commence- 

 ment of Silurian times, however, we find at once a considerable variety 

 of animal forms. 



What, then, was the character of this earliest fauna and flora ? If 

 we could have walked along that beach when it was washed by pri- 

 mordial seas, what would we have found cast ashore ? We would have 

 found the representatives of all the great types of animals except the 

 vertebrata. The Protozoa were then represented by sponges and 

 Ehizopods ; the Eadiates by Hydrozoa (graptolites) (Fig. 276) and Cys- 

 tidean Crinoids ; the mollusks by Brachiopods, Lamellibranchs, Gas- 

 teropods (Pleurotomaria), Pteropods (Figs. 272-279), and even Cepha- 

 lopods (orthoceras) ; and the Articulates by Crustaceans (trilobites, 

 etc.) (Figs. 269-271) and Worms (Fig. 281). Plants are represented by 

 Fucoids. These widely-distinct classes are already clearly differentiated 

 and somewhat highly organized. Nor is the fauna a meager one in 

 number of species. In the United States and Canada alone about 400 

 species are already known in the primordial, of which nearly 100 are 

 trilobites; and in the lowest zone of the primordial, viz., Olenellus 

 beds, there are 134 species, of which 55 are trilobites (Walcott). About 



Fig. 280. 



Fig. 283. 



Fig. 286. 



Fig. 288. 



Figs. 280-288.— Foreign Primordial Fossils: 280. Oldliamia antiqua, probably a plant. 281. 

 Arenicolites didymns, worm-tubes. 282. Lingulella ferruginea. 283. Theca Davidii. 284. 

 Modiolopsis solvensis. 2^5. Ortliis Hicksii. 286. Obolella sagittalis. 287. Hymenocaris vermi- 

 cauda. 288. Olenus macrurus. 



a dozen species of plants are also known. When we recollect the great 

 age of these rocks and their usual metamorphism, and the fragmentary 



