THE DEVONIAN PERIOD 133 



size, having attained a diameter of two or three feet. Diatoms are 

 unicellular, aquatic plants of microscopic size which secrete shells 

 of silica, and some of Devonian age are known. In some of the 

 later periods these tiny plants were of considerable importance. 

 No Bryophytes (Mosses) have yet been discovered. Spores and 

 spore-cases of certain aquatic plants (Rhizocarps) , probably related 

 to very simple Pteridophytes, are very abundant in the black 

 shales, especially those of Marcellus and Hamilton ages. Accord- 

 ing to Dawson they are " dispersed in countless millions of tons 

 through the Devonian shales," and by their decomposition much 

 oil has been produced. 



Our knowledge of land plants prior to the Devonian is very 

 scant as we have seen, but the records are sufficient to make it 

 certain that the Devonian lands were covered with a rich and diver- 

 sified vegetation, often even with luxuriant forests. The forests 

 were, however, far different in appearance from those of the present 

 because the trees were all of very simple or low organization types. 

 Thus they were largely represented by all the main subdivisions 

 of the non-flowering Pteridophytes such as Lycopods, Equisetce, 

 Ferns, and Seed-ferns, and some simple types of the lower order 

 of flowering plants (i.e. Gymnosperms) were also present. Since 

 these important and remarkable land plants reached their climax 

 of development in the Pennsylvanian (great coal period), it will 

 serve our purpose best to discuss these plants in connection with 

 the flora of the Pennsylvanian. 



Protozoans. — Foraminifers and Radiolarians no doubt existed 

 because they are known from the immediately preceding and suc- 

 ceeding periods, but fossil forms have not been found. 



Porifers. — Sponges were common but they require no special 

 description. 



Coelenterates. — The Graptolites which are so abundant and 

 important for correlation purposes in the three preceding systems, 

 are comparatively rare in the Devonian, and they became almost 

 extinct before the close of the period. 



Corals displayed a very marked increase in numbers, species, 

 and size. They must have grown in profusion, especially in the 

 clear Onondaga sea, as proved by the many great fossil Coral reefs. 

 From near Louisville, Kentucky, alone more than 200 species are 

 known, and these are only a fraction of all described Devonian 

 species. They were almost all of the cup and honeycomb types, 



