492 



HISTORICAL GEOLOGY 



by their absence, and even grasses and mosses were lacking. The 

 living relatives of the Carboniferous trees are for the most part lowly, 

 inconspicuous plants. 



The land plants of the Carboniferous belonged to six great groups : 

 (i) ancestral ferns, (2) seed ferns (pteridosperms), (3) club mosses 

 (lycopods), (4) an ancient extinct group, the sphenophylls, (5) the 

 horsetails (Calamites), and (6) the Cordaites and possibly some 

 conifers. 



(1) Ancestral Ferns and (2) Seed Ferns. — The shale overlying coal 

 beds is often full of the fronds of fernlike plants (Fig. 468), and so 

 perfect is the preservation of some of them that the finest venation is 



shown. The beauty of such fossils 

 is especially striking when the 

 shale is light in color, since under 

 such conditions the delicate out- 

 line of the black fossil frond is 

 brought out with great distinct- 

 ness. It was formerly thought 

 that these fernlike leaves were 

 the remains of ferns, but a more 

 careful study and further dis- 

 coveries have shown that few are 

 true ferns; on the contrary, the 

 great majority belong to an ex- 

 tinct family, the seed ferns or 

 pteridosperms. 



The important difference be- 

 tween these families lies in the 

 reproduction, the true fern pro- 

 ducing spores and the pteridosperms, seeds. 



Ancestral Ferns. — The most important family of Paleozoic ferns 

 (Marattiaceae) has descendants living to-day, but the Paleozoic 

 members of the family were tree ferns, reaching in some species a 

 height of upwards of 60 feet. This great family has dwindled to a 

 few genera which are now confined to the tropics ; one of which, how- 

 ever, the elephant fern, sends up huge fronds to a height of 10 or 12 

 feet. In addition to the tree ferns there were doubtless low, herba- 

 ceous ferns, living under the same conditions as the ferns of to-day. 



Seed Ferns (Pteridosperms). — The members of this extinct group, 

 if living to-day, would probably be called ferns by the casual 



Fig. 468. 



A Paleozoic fernlike plant, 

 Pecopteris. 



