12 CATALOGUE OF THE CRETACEOUS AND [bull. 152. 
that it was below well-defined marine Cretaceous, and hence in the 
Montana formation. 
Another source of uncertainty results from the disagreement among 
geologists and paleontologists as to the age of various beds. Thus, by 
some the Fort Union beds are regarded as Cretaceous and by others as 
Tertiary, and the so-called 4i Arctic Miocene" is coming to be regarded 
as Eocene. When it has been possible, without circumlocution, the 
horizon has been indicated in a way that will fix its relative position 
without going too far. Thus the use of the Belly Eiver series. Denver 
beds, Fort Union beds, Amboy clays, Potomac formation, etc., indicates 
sufficiently the geological position, without attempting a discrimina- 
tion perhaps unwarranted by the present state of our knowledge. But 
above all it should be remembered that this is an attempt at a cata- 
logue of our Cretaceous and Tertiary plants, and is not intended as a 
complete exposition of their geographical or geological distribution. 
Xo arbitrary signs or characters have been used in this work. In a 
few cases it was found necessary to give new names on account of the 
old name being preoccupied. In such cases the new name is followed 
by "nom. nov." In accordance with principles quite generally adopted 
among recent botanists, the attempt is made in this catalogue to 
restore the oldest specific name under which a species was described. 
This has resulted in a number of new combinations, which are indi- 
cated by the abbreviation "n. comb." In the case of two dates given 
for a publication (e. g., Fl. Amboy Clays, 1895 [1896]) the first is the 
title-page date, the bracketed date being the actual date of issne. 
The bibliography consists simply of a list of the Xorth American 
works and papers that have been consulted in preparing this catalogue, 
together with such others as contain original descriptions of our plants. 
It was not thought worth while to give a bibliography of the references 
to the numerous foreign species that have been identified in our Mora, 
with the single exception of those found in the various parts of Heer's 
Flora Fossilis Arctica. 
In conclusion. I beg to acknowledge my indebtedness to Prof. Lester 
F. Ward for valuable suggestions, use of books, etc.. and to his assist- 
ant, Miss C. H. Schmidt, for assistance in catologuing and verifying 
references. 
