252 



PALEOZOIC TIME. 



shales and sandstones, and this fact, together with the new incursion 

 of British species, is evidence that this eastern coast-barrier had 

 dipped down beneath the ocean again ; while the additional fact that 

 the rocks of the same Atlantic border, which follow the Trenton, in 

 the Upper Silurian era, are mainly limestones, would seem to prove 

 that the barrier was only partly submerged. 



Life The closing period of the Lower Silurian gave the first posi- 

 tive evidence of the existence of terrestrial plants in the world. As 

 stated in connection with the Archaean, on pp. 157, 158, the fact that 

 Lichens are not found fossil, nor the destructible Fungi, is no evidence 

 that these classes of terrestrial vegetation were not also well repre- 

 sented. Considering the millions of years that passed in the course 

 of the Lower Silurian and the first half of the Upper Silurian (nearly 

 half of all geological time from the commencement of the Primordial 

 onward), and the numberless chances for the burial of a drifted leaf, 

 or broken stem, or a whole uprooted plant, if any such existed along 

 the sea-shores, or in valleys which poured streams into the continental 

 seas, the absence of remains of land plants had appeared until 1878 

 to be strong presumptive evidence that they did not exist. But some 

 few specimens since observed show that the land of the Trenton period 

 had its Ferns and Lycopods like the Devonian and Carboniferous 

 eras. Such plants suggest that there were also species of terrestrial 

 articulates ; and they may yet be found. We have no right to sup- 

 pose Mosses to have then existed, since none is known from the Devo- 

 nian and Carboniferous beds. 



The animals of the Upper Silurian, found fossil in American rocks, 

 are all Invertebrates, like those of the Lower Silurian, and similar 

 in general types. The Cephalopods are the highest species among 

 Mollusks, and the Trilobites or Eurypterids among Crustaceans. But, 

 in Great Britain and Europe, the existence of Fishes is made certain 

 by various fossils ; and these Fishes were either of the tribe of Sharks 

 or of that of Ganoids. 



Among the Invertebrates, there was constant change, some groups beginning, others 

 expanding to their climax, and others disappearing. Graptolites, which passed their 

 climax in the Lower Silurian, had comi aratively few species in the Upper. Crmoids 

 and Corals were brought out in various new forms, and of increasing, variety. The 

 Chain corals (Halysites) are an example of a genus that ended in the Upper Silurian, 

 while the Favosites and Cyathophylloids are more multiplied in after time. 



Mollusks were still most abundantly represented by Brachiopods. The genera Spi- 

 rifer, Athyris, Chonetes, Rensselaeria, and others, were added to Lingula, Orthis, Lep- 

 ttzna, Rhynchonetta, Atrypn, etc., of the Lower Silurian; at the same time, Orthis had 

 lost its preeminence, and was of few species. The Lower Silurian Brachiopods have 

 no bony arm-supports internally, excepting those of Atrypa and Rhynchonella. In 

 both Spirifer and Atrypa, these supports were long and rolled spirally. The genus 

 Spirifer commenced with narrow species, little broader than high; but in the later part 

 of the Upper Silurian they were already much wider, though not as extravagantly so 

 as in many species of the Devonian and Carboniferous. In the Niagara occurred the 



