ANCIENT LIFE ON THE EAKTH 31 



obscure, so that it no longer resembles its ancient 

 ancestors. 



The Eurypterida are closely related to the Xipho- 

 sura, but show also remarkable affinities to the 

 Scorpions. Behind the small head-shield they have 

 twelve free abdominal segments which bear no 

 appendages except, perhaps, gills. They were the 

 largest and most powerful animals of their day, 

 some of them attaining a length of nearly six feet. 

 They obtained their maximum in the Silurian 

 period, and became extinct before the end of the 

 Carboniferous. 



Of plants we have at last certain knowledge. Not 

 only have numerous impressions been found in 

 Silurian rocks, which appear to have been made by 

 seaweeds ; but even as low down as the Ordovician 

 there is a land plant, called Buthotrephis , which is 

 closely related to Annularia; and another, called 

 Protostigma, related to Sigillaria. In the Silurian 

 period we find the extraordinary Nematophycus, 

 which appears to have been a terrestial alga, but as 

 tall as a tree ; and there were two genera of Lycopo- 

 diacece (Psilophytum and Glyptodendron) , and at 

 least two genera of Equisetacece (Annularia and 

 Sphenophyllum). Ferns, apparently, were not yet 

 in existence, for the so-called Eopteris is now known 

 to be nothing more than a growth of dendritic 

 crystals. 



Land Animals. In the Ordovician of Sweden the 

 remains of an insect (Protocimex) , belonging to the 

 Hemiptera, has been found. And in the Silurian 

 of France there is Palceoblattina douvillei, which, 



