FOX HILLS AND LOWER MEDICINE BOW 27 



In the interprctation and application of the data presentcd in table 2, it is 

 evidcnt in the first placc that on the basis of margin charaeters the Aledicine Bow 

 association is comparablc to forests approaching the subtropical in aspect. The 

 large numbcr of entire-leaved spccies (67%) is only 4% less than Bailey and Sin- 

 notfs lowest figure for sul)tropical forests. Among fossil fioras the Mediciiie Bow 

 asscmblage is of decidedly warmcr aspect than the floras of the Bridge Creck (15%), 

 the Green River (29%), and the Florissant (35%); the Wilcox flora, on the other 

 hand, appears to be more subtropical (83%). The Goshen and Laramie floras are 

 more nearly similar to the Medicine Bow in percentage of entirc-leaved species. 

 The Goshen flora (61%), according to Chaney and Sanborn.i indicatcs a humid, 

 subtropical climate. The Laramie flora (71%) has unfortunately not as yet been 

 thoroughly studied with a view to accurate postulation of climatic conditions. 



In the column under " Length" in table 2, the percentage of large leaves in the 

 Medicine Bow flora shows likewise its closest relationship to that of the Goshen 

 flora. In nervation characters a closer similarity is shown to the Weaverville 

 flora, which MacGinitie ^ has described as a warm temperate association, com- 

 parable to the modern flora of southern Alabama. Although the remaining leaf 

 cbaracters are not regarded as sufficiently trustworthy criteria for individual con- 

 sideration, they tend to substantiate the similarity of the Medicine Bow species 

 to those of the subtropical Goshcn flora on the onc hand and the warm temperate 

 Weavervillc flora on the other. 



To summarize the inferences made on the basis of reliably tested leaf charac- 

 ters, it is evident that the Medicine Bow flora is distinctly not a cool temperate 

 assemblage, but is intermcdiate in character between warm temperate and sub- 

 tropical, morc nearly approaching the latter. 



PAST CLIMATIC CONDITIONS AS INDICATED BY THE FLORA 



In addition to climatic studies based upon leaf characters, environmental con- 

 ditions of the past may also be cstimated by comparing thc fossil species of a given 

 flora with their nearest living relatives; it may be inferred that the past conditions 

 were analogous to those under which the majority of living relatives grow. 



The application of this method to the lower Medicine Bow flora encounters 

 several difiiculties. In the first place, many of the fossil species are not generically 

 comparable to any living forms which have come to my attention. This may 

 mean one of at least three tliings: (1) these genera may not have survived from 

 the Upper Cretaceous to the present; (2) they may be ancestral types from which 

 descendants with somewhat different leaf characters have since evolved; or (3) they 

 may belong to genera with which I am unfamiliar. In the second place, even when 

 identifications are more certain, it cannot be estabhshed that in the Upper Creta- 

 ceous a gcnus responded to climatic influences precisely as it does today. This 

 difficulty is in a large measure overcome by placing less stress upon individual 

 genera in the fossil flora and more upon the flora as a whole. Thirdly, where close 



' Chaney, R. W.. and Sanbom, E. I.. Carnegie Inst. Wash. Pub. No. 439, 57, 1933. 

 2 MacGinitie, H. D.. Carnegie Inst. Wash. Pub. No. 465, 113, 130, 1937. 



