208 



PALAEOZOIC TIME — LOWER SILURIAN. 



Specimens of Sea-weeds are rare. Fig. 262 is the Bxithotreph is gracilis, and 

 fig. 263, B. sxiccidosus. The figures represent only portions of these plants. 

 Many fossil Sea-weeds are not to be looked for in limestones. 



62 



Figs. 262, 263 



Fig. 262, Bxithotrepkis gracilis ; 263, B. succulosus. 



2. A, 



ils. 



The seas of the Trenton period were densely populated with 

 animal life. Many of the beds are made of the shells, corals, and 

 crinoids packed down in bulk ; and the less fossiliferous compact 

 kinds have probably the same origin, and differ only in that the 

 shells and other relics were pulverized by the action of the sea, and 

 reduced to a calcareous earth before consolidation. 



With the Trenton period there appeared species of undoubted 

 Polyps, the true coral animals of, the seas (fig. 277, etc.) ; and the 

 sub-kingdom of Eadiates has hence all its three classes, Polyps, 

 Acalephs, and Echinoderms, represented. These corals belong 

 mostly to the Cyathophyllum family, and where they occurred they 

 gave the aspect of a flower-garden to the sea-bottom in shallow 

 waters. The Molluscan sub-kingdom included numbers of Con- 

 chifers and Bryozoans, as well as Brachiopods, Pteropods, Gastero- 

 pods, and Cephalopods ; and all these divisions were well repre- 

 sented. Both the Molluscan and Eadiate sub-kingdoms were, 

 therefore, fully unfolded in all their grand types, though not ad- 

 vanced to the high rank they afterwards attained. In the sub-king- 

 dom of Articulates there is no progress above Worms and Crus- 

 taceans ; and no trace of a Fish or of any Vertebrate has been 

 found in the rocks of the period. 



The prevailing types are— (1) Brachiopods (figs. 268-270, 286-300), 

 whose shells outnumber and outweigh all other remains together ; 

 (2) Orthocerata (figs. 313-315), of the class of Cephalopods, which 

 are numerous, and some of them ten to fifteen feet long and a foot 

 in diameter ; (3) Crinoids (figs. 264, 284, 285), which rank next to 



