
No. 464.] STUDY OF THE SALICACE $15 

Our knowledge of the fossil Salicaceze had its origin in the 
work of Heer upon the fossil plants from the Cretaceous forma- 
tion of Greenland, through his description of Populus primeva. 
The occurrence of this species in the Kome beds not only 
afforded the first real evidence of the occurrence of these plants 
in previous geological time, but it at once served to suggest the 
essentially primitive character of the Salicacee as a whole 
among Dicotyledons—an idea subsequently strengthened by 
the discovery of both Salix and Populus in the Potomac forma- 
tion, as well as in the Kootanie beds of Montana. This idea, 
then, has persisted to the present day and finds repeated ex- 
pression in the various treatises on fossil plants, although the 
acceptance of such a view is not based in any case, upon trust- 
worthy facts derived from a critical comparison of distribution, 
a knowledge of the phylogeny of the group, or an acquaintance 
with the anatomy of either recent or extinct species. Paleonto- 
logically it is of interest to determine the position of the Sali- 
'cacez relatively to the other families associated with it in the 
Cretaceous and Tertiary deposits, and for this purpose the flora 
carefully compiled by Dr. Knowlton (98) offers the most 
reliable basis now available. Among forty-five families of 
angiosperms now known to constitute the flora of the Creta- 
ceous and Tertiary ages in North America, it will be found that, 
‘geologically speaking, the Salicaceze is by no means the primitive 
group which seems to be implied by the position usually assigned 
to it, but that it really occupies a position which is the twenty- 
fourth in a series based upon the percentage ratio of occurrence 
in the two great geological periods. Such a series has as its 
lowest member, the Proteacez which is preéminently a Creta- 
ceous family, bearing to the Tertiary the ratio of 19:0. The 
same is also true of the Menispermacez with a ratio of 17:0, 
closely followed by the Araliacez (6:0) and the Euphorbiacex 
(2:0). Atthe other extremity of the series we find such fami- 
lies as the Hydrocharidaceze (0:2), Lemnacez and Simarubacez 
(0:3), Typhacez (0:4), Onagracez (o: 5), Naiadacex (0:7), 
Zingiberaceze (o : 17), and the Cyperaceze (o : 20), all of which are 
manifestly typical Tertiary plants. About midway of the series 
the Salicacez are associated with the Urticacee with a ratio of 
1: 1.47 and with the somewhat similarly distributed Platanacez, 
