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 Upper Silurian gives the first 

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

 stated in connection with the Archasan, 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 well represented. 

 But, 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 all higher land plants affords a strong 

 presumption that they did not exist. What were the precursors of 

 the Oriskany and Ludlow Lycopods and Gymnosperms, is yet wholly 

 unexplained. We have no right to suppose them to have been 

 Mosses, since no moss has yet been found fossil, even in the rocks of 

 the long Devonian and Carboniferous eras which follow. The species 

 of Eophyton (p. 176) are too doubtful to be here considered. 



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 comparatively few species in the Upper. CrinoidS 

 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 Lvngula. Orthis, Lep- 

 tarna, Ehynchonella, Atryp", 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 Ehynchonella. 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 



