66 THE LESSON OF EVOLUTION 



joined in the Silurian. All the Xiphosura of the 

 Palaeozoic era show a remarkable resemblance to the 

 Trilobites, as also does the young of the modern 

 Limulus, or King Crab. But as the latter gets older 

 the cephalic shield becomes united to that of the 

 thorax, and seven of the abdominal segments also fuse 

 together in an abdominal shield; while the primary 

 division into three longitudinal lobes gets 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 appen- 

 dages 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 terrestrial alga, but as tall as a tree ; and there 

 were two genera of Lycopodiacece (PsHophytum and 

 Glyptodendroii), and at least two genera of Eguisetacece 

 (Annularia and Spkenophyttum). 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. 



