490 GEOLOGY. 



thodians, sharks amply armed with fin-spines (such as Climatius (Fig. 

 225) Diplacanthus, Cheir acanthus, etc.). There does not appear to be 

 any clear evidence that the pavement-toothed sharks occupied the 

 fresh waters, and this is not strange since these probably did not offer 

 enough shell-fish to be inviting feeding-grounds. 



The associated arthropods. — In view of the arthropodan rela- 

 tions of the ostracoderms, it is suggestive that now again, as in the 



Fig. 225. — Climatius scutiger, natural size, from Old Red Sandstone, Scotland. 

 (From Zittel, after Powrie.) 



Salina and in the transition beds of the European Silurian, there were 

 gigantic eurypterids associated intimately with the ostracoderms and 

 the fishes. Taken together, these constitute a most remarkable fauna. 

 The Pterygotus now reached the extraordinary length of two meters. 

 There were also ostracodes, phyllocarids, and phyllopods related to the 

 Estheria of modern brackish waters. On the land, too, were scorpions, 

 probably related ancestrally to the eurypterids, as well as their 

 not distant kin, the spiders, together with myriapods and insects, 

 probably also ancestrally related to some of the aquatic crustaceans. 

 It has already (p. 482) been suggested that this association of arthro- 

 pods and vertebrates ran back to their origin, and it may here be 

 added that it ran on until the present day, for the fish and the arthro- 

 pods (crayfish and smaller crustaceans) dominate the fresh waters, 

 while the terrestrial vertebrates and the insecteans are only too inti- 

 mate associates and contestants on the land. 



Fresh-water mollusks. — Shells believed to have belonged to fresh- 

 water mollusks, and closely resembling living genera, have been found 

 in England, Ireland, Russia, and the United States, accompanied by 

 land plants and by fishes. Some traces of what are supposed to have 

 been fresh-water p^nts have been found, but as yet little is known 

 of the aquatic vegetation. 





