Eozoic Age. 73 



forming the carbon beds of the Laurentian times. Some of the diatoms 

 surround themselves with shells of siliceous material, which are known 

 to furnish a sufficient accumulation in places, to form layers of flint}- 

 stone. Nullipores and Corallines are simple plants that also form de- 

 posits of calcareous matter, which are known to contribute to the f orma- 

 tion of limestone beds. There are many large forms of algae, all, how- 

 ever, of very simple organic structure, some of which, chiefly fucoids, 

 a leathery variety, are found in the Potsdam period. We are at liberty 

 to conclude that the simplest forms of both animal and vegetable life 

 existed for many long ages before the beginning of Silurian times. 



CHAPTER IX. 



SILURIAN-AGE OF MOLLUSKS. 



LOWER SILURIAN AGE. 



The lowest or earliest part of this age is called the Potsdam period. 

 The only plant life in this age that made any record, consists of several 

 varieties of the fucoid algae. These are round stems, some thin, others 

 as much as half an inch in diameter. The latest of them are branched 

 varieties, yet they are all very simple in structure. They have no roots 

 or leaves, but absorb their nourishment through their cell walls. No 

 land plants are found and probably there were none But there must 

 have been immense quantities of the sea-weed, because there were im- 

 mense numbers of sea animals that depended upon it for support. 



This animal life was, within certain limits, greatly varied. Of zoo- 

 phytes there were sponges, graptolites and sertularians, and in the lat- 

 ter part of the period, corals. All these animals are single celled, liv- 

 ing in aggregations or communities, and attaching cell to cell, after a 

 pattern peculiar to each species. Most of them are like bushes or trees 

 of various patterns. 



Of worms there were a good many. The bodies of soft worms would 

 obviously not be of easy preservation, and it may be taken for granted 

 that many existed of which no vestige is left. ' ' Impressions of long 

 marine worms have been reported from some of the shales. Besides 

 these, there are worm-holes in the Potsdam sandstones though now 

 filled with rock which are referred to burrowing worms of the Arenicola 

 family ( so called from the Latin arena, sand, and incola, inhabitant). 

 They penetrate the rock vertically, and are often in pairs, as is now the 

 habit of such worms. The most common kind in the Potsdam sand- 

 stone is called scolithus linearis." ( Dana 185.) 



There were great numbers of brachiopods. These are a bivalve shell 

 fish, classed generally as the lowest order of the mollusks, and are called 

 molluscoids by some. In Hseckel's classification they are the spiro- 



