304: 



PALEOZOIC SYSTEM OF ROCKS. 



Plants. 



With the exception of a few small land-plants, ferns, and club- 

 mosses, recently found in the Middle Silurian of both this country and 

 Europe,* and of which we shall speak again, the only plants yet found 

 are the lowest forms of cellular cryptogams, viz., marine algce or sea- 

 weeds. It is difficult, from the impressions left by these to determine 

 genera, much more species, with any degree of certainty. We shall, 

 therefore, call them by the general somewhat indefinite name of Mu- 

 coids (Fucus, tangle or kelp), or Fucus-like plants. As already stated, 

 plants are far less abundantly and perfectly preserved than animals, on 

 account of their want of a skeleton. 



Animals. 



Protozoans. — The large, irregular masses which are called Eozoon 

 seem entirely characteristic of Archaean times. If they are indeed of 



animal origin, they 

 are replaced in the 

 Silurian age by more 

 regular forms which 

 are usually called 

 sponges. Of these, 

 the most character- 

 istic Silurian genera 

 are Stromatopora 

 and Eeceptaculitis 

 (Figs. 297 - 303). 

 They seemed to have 

 formed large coral- 

 line masses,which are 

 now regarded either 

 as hydrocorals (Stro- 

 matopora) or as com- 

 pound Khizopods 

 (Receptaculitis). 



Radiates, Corals. — Corals were very abundant, forming often whole 

 rock-masses, as if they, while living, formed reefs. These, if they in- 

 dicate warm seas, show a great uniformity of temperature, since they 

 are found in all portions of the earth alike. 



The corals of the Silurian age belong principally to three families, 

 viz., Cyathophylloids, or cup-corals ; Favositidaz, or lioney-combed cor- 

 als ; and Halysitidm or chain-corals. They are remarkable in not 



Fig. 297.— Stromatopora rugosa. 



Lesquereux, American Journal of Science, 18*78, vol. xv, p. 149. 



