46 AN INTRODUCTION TO THE STUDY OF FOSSILS 



The Pteridophyta are sub-divided into the following orders : 



a. Filicales (including Ophioglossales, which is sometimes con- 



sidered a distinct class), — the ferns. 



b. Equisetales, — the horsetails. 



c. Lycopodiales, — the club-mosses. 



d. Sphenophyllales. 



ORDER a, FILICALES 



The ferns usually possess a broad frond or leaf which is 

 often divided into pinnae, or leaflets. The spore cases are 

 gathered into sori, the round " fruit dots," borne on more or 

 less modified portions of these leaves or on independent fruiting 

 stalks. Common living examples are Polystichum, — the 

 Christmas fern, and Onoclea, — the sensitive fern. 



Ferns are known to have existed from the time of the Devo- 

 nian, and must have appeared before the close of the Silurian. 

 Nearly all the living families have existed since the Jurassic, 

 while at least two families, the Osmundaceae and the Maratti- 

 aceae. have been traced back to Paleozoic ancestors. It is rare 

 that all of the organs of any fossil plant may be described, since 

 they are so easily separable from one another. This incomplete- 

 ness of data is, for example, true in the case of fern-like forms. 

 Hence until recently the characterization of many fossil genera 

 rested solely on the form and venation of the leaf. One of the 

 largest of these frond genera w^as Neuropteris. Fructifications 

 later found in connection with leaves of this genus have deter- 

 mined that this as well as many other genera with fern-like fronds 

 is not a true fern but a member of the great group of Cycado- 

 filicales, — an order of Pteridophyta intermediate between the 

 ferns and the cycads. 



Fossil examples of true ferns are : 



(i) Osmundites (Jurassic to Tertiary). — Fern stems with 

 the peculiar structure of the stem of the living Osmunda, — - 

 the common royal fern. This relationship is suggested by the 

 name in which the suffix ''-ites " added to the name Osmunda 



