In Silurian and Devonian Epochs 97 



already advanced weighty evidence for the view that all 

 the marine organisms were secondarily derived from fresh- 

 water types that had previously evolved and lived amid 

 freshwaters. These, he claims, sent derivative offshoots 

 into littoral marine regions, which there fed and multiplied 

 abundantly. Partial proof of this is that nearly all of 

 the main invertebrate groups retain their most primitive 

 types in freshwater environment up to the present day, 

 (7:378-410). 



But as we pass through the series of Silurian rocks a 

 striking change can be traced. By the earlier students of 

 these rocks their uniformly marine deposition was accepted 

 without question. And so far as the Lower Silurian or the 

 Llandovery and Wenlock beds of the Upper Silurian reveal 

 their secrets, their fauna is probably wholly marine. The 

 succeeding or Ludlow Rocks, however, indicate a marked 

 change over wide areas, alike of Northern Europe and 

 North America, and such can now be treated under the 

 succeeding caption. 



A. The physical and biological environment of Silurian 

 Fishes. 



We may begin with the now well known central English 

 rocks that are classed collectively as the Ludlow group, 

 since increasing study has been given to them and their 

 enclosed organisms in that region from about i860 onward. 

 First described by Murchison in 1826 {28: 12), and more 

 fully in "Siluria." {26: 137) two decades later, they were 

 examined in detail by Symonds (52: 193), then by Symonds 

 and Lambert (55:152) who described the exact succes- 

 sion and thickness of the beds, as determined during con- 

 struction of the Malvern and Ledbury Tunnels. This 

 succession beginning from below, deserves quotation — 



"1, Aymestry rock with Pentamerus Knightii, etc. (lo ft.) 



2. Upper Ludlow rock with Chonetes lata, etc. (140 ft.) 



The Ludlow bone-bed seems to be wanting here. 



3. Downton bed, thin (9 ft.) with Lingula. 



4-8. Red and mottled marls and thin sandstone (210 ft.) with Lin- 

 gula and Pteraspis. 

 9. Gray shale and thin grit (8 ft.) with Cephalaspis and Ptery- 

 gotus. 

 lo-ii. Purple shales and thin sandstones (34 ft.)- 



12. Gray marl passing into red and grey marl and bluish gray 

 rock (20 ft.), with Auchenaspis, Plectrodus, Cephalaspis, 

 Onchus, Pterygotus ludensis, Lingula and a Lituite (?). 



