12 PENNSYLVANIAN FLORA OF NORTHERN ILLINOIS 



has kept in mind the biologic considerations as they are expounded in the 

 most recent general paleobotanic textbooks such as the following: 



Seward, A. C: Fossil Plants, 4 vol., London, 1898-1919. 



Berry, E. W. : Paleobotany, a Sketch of the Origin and Evolution of Floras, Washington, 

 1920. 



Potonie, H. and W. Gothan: Lehrbuch der Paleobotanik, 2nd edition, Berlin, 1921. 



Scott, D. H.: Studies in Fossil Plants, 3rd edition, London, 1921-23. 



Gothan, W.: Leitfossilien-Karbon und Perm. Pflanzen, Berlin, 1923. 



Scott, D. H.: Extinct Plants and Problems of Evolution, London, 1924. 



Acknowledgments 



The author wishes to express his sincere thanks to D. C. Young, the 

 donor of a beautiful collection of fossil plants from the Mazon Creek 

 deposits to the University of Chicago; to Ed. Carr, son of the collector, 

 J. C. Carr, for many courtesies and numerous fossils given to the University 

 of Chicago; to F. C. Baker, Curator of the Natural History Museum, 

 University of Illinois, for his great kindness in facilitating the study of 

 the Carr and Daniels collections; to W. J. Knoblock in Quincy and George 

 Condie in Spring Valley for the loan of valuable fossils; to Dr. W. E. Walsh 

 in Morris for specimens and valuable assistance; and to W. Oswald of 

 Braidwood for his courtesy and help. 



The deepest gratitude of the author is felt toward the State Geological 

 Survey of Illinois which has facilitated and published these studies. Special 

 thanks are also due to the former Chief of the Survey, Mr. F. W. DeWolf, 

 and to the present Chief, Dr. M. M. Leighton, who authorized and gener- 

 ously supported these investigations. The author is much indebted to 

 Dr. Harold E. Culver, who was in charge of the coal investigations of the 

 Illinois State Geological Survey and who gave invaluable aid throughout 

 the investigation. To Dr. David White, who has critically read this report 

 and offered most valuable suggestions, the author wishes to express his 

 sincere appreciation. 



IDENTIFICATION OF FOSSIL PLANTS 

 Definition of a Fossil Plant Species 



In order to identify and classify fossil plants, it is important to have 

 in mind what a fossil species is. It is more difficult to distinguish fossil 

 plant species which are fragmentary than to recognize living plant species 

 where complete specimens are available. It has occurred frequently that 

 two fossil leaf forms have been described as representing two distinct 

 species and similar forms were later found connected on one single branch. 

 Such fossils as show characteristic differences and have never been found 

 to have grown on one and the same plant should be considered as differ- 

 ent species. 



If we keep in mind the pronounced polymorphism of many organs in 

 living plants, and assume that similar conditions prevailed in the Paleozoic 

 era, we must conclude that our species distinctions of fossil plants are only 

 approximately true and subject to constant revision upon the discovery of 

 more complete material. It also follows that the better known species 

 present a wider range of variations than those of which we know little. 

 Furthermore, we know most fossil plant species only from impressions and 



