THE PLATANUS OR BUTTONBALL 159 



Alabama and a third in the Montana Group of Utah. These 

 American records total 16 forms. The only other Cretaceous 

 records known to me are the not certain identification of 2 Dakota 

 Group forms from the Upper Cretaceous of Argentina, 1 cer- 

 tainly not a Platanus, and 3 supposed species from the Ceno- 

 manian of Bohemia described by Velenovsky and Marik, The 

 latter I regard as referable to the probably alHed genus Credneria 

 of Zenker. 



Regarding the authenticity of the botanical determination of 

 these various species it is probable that some of the records are 

 worthless, but enough remain which are based upon an abundance 

 of absolutely characteristic leaves, in some cases accompanied by 

 typical fruits, to render it certain that in Middle Cretaceous times 

 ancestral plane trees were an abundant element in the flora of 

 North America, and that later in the Cretaceous they had spread 

 to South America and the Arctic region. They may have con- 

 tinued across the latter region into Europe although the records 

 are not entirely convincing as regard the Cretaceous but are more 

 ample in support of such a migration in the Tertiary. 



These early ancestors had somewhat elongated rhomboidal 

 leaves, with irregularly and remotely toothed margins, decurrent 

 on the petiole, which was conspicuously enlarged at the base. 

 There was a tendency, not especially pronounced, toward palmate 

 triloba tion. The floral axes were already shortened and aggre- 

 gated and the fruiting heads were racemose as is indicated by the 

 predominancy of this habit in the existing species and its frequent 

 occurrence in forms like Platanus occidentalis that normally have 

 but one fruiting head to a peduncle. These ancestral leaf char- 

 acters are deduced from the form of the earhest species and from 

 the substantial agreement between them and the leaves of modern 

 seedlings and adventiLive shoots from old stumps both of which 

 are supposed to exhibit more or less reversionary characters. 

 Several of these are figured in the present connection. 



Reacting to the genial influences of the Cretaceous climate these 

 early forms soon broadened their leaves, which also became 

 lobate, so that Platanus Kilmmeli Berry of the Magothy Forma- 



