PENNSYLVANIAN FLORA OF NORTHERN ILLINOIS 13 



must base our species descriptions on few and often secondary charac- 

 ters. We have form types of this case — such as, stem types, leaf types, and 

 fructification types. Only in comparatively few instances do we possess 

 petrefacts with well preserved tissues and, under favorable circumstances, 

 can we establish phylogenetic, that is, evolutionary relations and distinc-. 

 tions. Under such favorable but rare circumstances we actually have 

 biological species types of the same value as offered by living plants. 



The plants that are figured in this report fall under the following 

 general groups: 



1. Equisetales 



2. Sphenophyllales 



3. Lycopodiales 



4. Filicales 



5. Cycadofilicales 



6. Cordaitales 



Groups 1 to 4 inclusive belong to the general phylum of Pteridophytes, 

 while groups 5 and 6 belong to the Gymnosperms of the Spermatophytes. 

 No Angiosperm impressions were recognized in the Paleozoic era, although 

 they probably existed, because Angiosperm-like structures were found in 

 coal balls from the Pennsylvanian of Illinois 1 . 



The six groups mentioned above, with the exception of the Filicales, 

 represent truly phylogenetic divisions of the plant kingdom. Originally 

 all fern-like fossil leaves were classed as true ferns until many of them were 

 found connected with Gymnosperm-like seeds. Early in the twentieth 

 century the Cycadofilicales or seed fern group was established and was 

 joined to the Gymnosperms. Since in most cases we deal with fern-like 

 leaf impressions which may phylogenetically belong either to Pteridophy- 

 tes or Gymnosperms, the most convenient form of classification may be 

 Filicales or, as Potonie had suggested, Pteridophylls. As the latter name 

 has not yet been introduced in this country, we may still be permitted to 

 call the group Filicales, but it must be kept in mind that the fossils classified 

 under this name are only a form group, an assemblage of leaf types, which 

 may belong to true ferns or to seed ferns or, in some instances, to both. 



List of Figured Species 



The following forms, classified according to the various groups, were 

 found in northern Illinois and are illustrated in Plates I to XLV: 

 Equisetales: 



Calamites suckowi Brongniart 



Annularia stellata (Schlotheim) Wood 



A. radiata (Brongniart) Sternberg 



A. sphenophylloides (Zenker) Gutbier 



Calamocladus equisetiformis (Schlotheim) Schimper 



Calamostachys solmsi (Weiss) Weiss 



C. germanica Weiss 



C. magna Lesquereux 



Joskins, J. H., A Paleozoic Angiosperm from an American coal ball; Botanical Gazette: pp. .590- .597, 1923 

 Noe, A. C, A Paleozoic Angiosperm; Journal of Geology: Vol. xxi, No. 4, pp. 34-4-^47, 1923. 



