INTRODUCTION 1 5 



diagnoses are given provisionally, since it is quite probable that 

 in consequence of the fragmentary nature of the available mate- 

 rial future studies will show several supposed species to be really 

 identical with one another. In the case of a large number of 

 fossil species it has not been found possible to obtain the type 

 sections for descriptive purposes. Under these circumstances, 

 although the original diagnoses differ materially from the general 

 plan adopted, it has been thought best to incorporate them in 

 the form of first publication, but with the name of the author 

 appended. 



All of the illustrations employed in the preparation of the 

 present work have been prepared from drawings and photographs 

 by the author, and they may therefore be regarded as being par- 

 ticularly applicable to the various questions brought under dis- 

 cussion. Nearly all the text figures were first published in the 

 American Naturalist, and are here reproduced through the cour- 

 tesy of that journal. Some of the half-tone reproductions of 

 photomicrographs appeared in earlier papers relating to fossil 

 and recent gymnosperms, while yet others are introduced here 

 for the first time. It has been impossible to introduce all the 

 illustrations which the clearest exposition might make desirable, 

 owing to the limitations imposed by the expense of such a pro- 

 ceeding ; but it is felt that the very generous allowance made by 

 the publishers in this respect will suffice to render the leading 

 facts of structure and relationship clear enough for our present 

 purpose. 



