Summary of Conclusions Reached 507 



Chapter 4. The physical and biological environment of 

 fishes during the Silurian and Devonian 

 periods. 



After review of earlier writings on the Upper Silurian 

 of Central England the author concludes that the abundant 

 primitive fish remains, crustaceans, eurypterids, and the 

 alga Pachytheca are a freshwater association, and that 

 marine organisms are wholly absent, the broken shells of 

 Lingula etc. being wash-outs from older deposits. The 

 Downtonian beds of Scotland, of W. Russia, of S. Norway, 

 of E. Canada and of Pennsylvania are similarly regarded. 

 Their fish remains, including many "conodont" organisms, 

 were primitive types derived from evolved metanemertean 

 ancestors. 



In study of the Devonian rocks fish-life is shown to have 

 inhabited only the wide river, flood-plain, lake and swampy 

 areas whose deposits were known as the Old Red formation. 

 Descendants of primitive Upper Silurian fishes swarmed 

 in these retreats, and have left their remains as teeth, scales, 

 plates, or spines. Sudden and wide-spread destruction also 

 of these — specially in the Upper Old Red period — gave 

 rise to the bone-beds, fish-beds, and petroliferous shales 

 of the Old World and the New. 



In these Old Red freshwater deposits ancestral types of 

 the Cyclostomata, the Elasmobranchs, the Dipnoans and 

 the Ganoids were abundant. None are met with alongside 

 marine organisms in marine beds of the period. Compari- 

 son also of Old Red remains from W. Russia, Bohemia, 

 Britain, Hudson's Bay, Ellesmere Land, and westward 

 to Iowa, and at least from Australia southward to Ant- 

 arctica, shows that like genera and Identical or nearly allied 

 species extended over freshwater areas that must have 

 reached almost from Pole to Pole. 



Chapter 5. The physical and biological environment of 

 fishes during the Carbo-Permian periods. 



The Calclferous or Lowest Carboniferous beds of Scot- 

 land are shown to be almost wholly of freshwater origin, 

 and in their strata — oil shales, freshwater limestones, shaly 

 sandstones etc. — to contain remains of abundant plants, 



