232 THE FLORA OF THE DAKOTA GROUP. 



enlarged into a round dentate pelta, traversed by the petiole. It is a leaf 

 of Aspidiophyllnni l)y the general outline and the contracted base, while the 

 basilar nppendage or ])elta is like a primordial form of the stipules, as in 

 riafatiiis hdnUobata Ward, of the Laramie Group, /'. oppend'K iilata Lesq., of 

 the auriferous gravel foruiation of California, and definitively in P. occi- 

 dentalis of the living flora. The leaves of this last species preserve mostly 

 the chai-acters already remarked in those of the Dakota Group and described 

 as belonging to Platanus prinKBva, though the first specimen desci-ibed 

 (Lesquereux, Cret. Fl., PI. vii. Fig-. "2) was only a mere fragment, and has 

 in its outlines some likeness to Credneria. Now we have in PI. VIII, 

 together with an entirely preserved leaf of the same type, a raceuie of 

 flowers evidently warranting its reference to Platanus, and in PI. IX two 

 varieties of leaves equally distinctly dentate like those of P. aceroides of the 

 Miocene, and of the living F. occidentalis. The same remark on the varia- 

 bility of the leaves of the Dakota Group might be made in comparing the 

 forms and characters of those which have been described as Aralia, Ster- 

 culia, Cissites, Menispermites, Pi'otophyllum, etc. The transformations, 

 however, are not always so distinct or so widely difl'erent. In Aceritcs 

 midtiformia (PI. XXXIV, Figs. 1-9), for example, if we comjjare the extreme 

 forms, that of Fig. 1 and Fig. 4, the leaves seem to represeut two well 

 characterized species. But in pursiiing the comparison tlu-ough the inter- 

 mediate forms, one can not say where to break the chain of relation for 

 the introduction of a new species. The leaves in this case evidently show 

 a disposition to metamorphosis, but it is limited to a certain deg-ree or to 

 gradual modifications, comparable indeed to what is observed sometimes at 

 our epoch among the leaves of a single tree. 



It is difiicult to imderstand what influences have acted upon the plants 

 of the Cretaceous in producing the transformation recog-nized in the appear- 

 ance of the first dicotyledonous leaf But it is rational to admit that this 

 influence, once in activity, has been continued and has rapidly undtijtlicd 

 and diversified the organization of the first rejiresentatives of the l)ic(»ty- 

 ledons. 



But how is it that, though the vegetable types are so easily and so 

 diversely modified near their origin, that the essential characters of many 

 of them remain persistent and may be recognized in the jjlants of more 

 I'ecent periods, being thei'e traced by their generic representatives and 

 even recognized in the flora of the present epoch ? I luive already as- 

 serted that most of the types of the arborescent fl<ira of North America 



