JURASSIC FLORA OF DOUGLAS COUNTY, OREG. 121 



fications of the rather polymorphous species Ginkgo digitata (Brongn.) 

 Heer. The Umits of variation in the same species, for leaves of the 

 Ginkgo type, are not fully known. Mr. Seward and Miss Gowan, in 

 their paper on Ginkgo biloba," have shown that the living Ginkgo tree 

 is capable of considerable variation in its leaves, and that some of these 

 forms even resemble Jurassic types. While forms of the leaves of the 

 same species may be selected that would show such a variation, the 

 significance as fossils of these variant forms would be better understood 

 if it were known what proportion in numbers they bear to the normal 

 forms and in what number they would occur in a fortuitous collection 

 of leaves produced by a fall from the same tree. 



In the Oregon localities many portions of rock are covered with 

 leaves of different types in about equal proportions, indicating that 

 each type belonged to different trees, on which it was the predominant 

 form of leaf. The greater number of the Oregon Ginkgo leaves may 

 be brought under several types of previously described forms. But 

 these may be connected by intermediate forms, which occur in con- 

 siderable numbers. Most of the previously described types illustrated 

 in the Oregon Ginkgos are those given by Heer for the Jurassic of 

 Siberia. It will perhaps give a better idea of the Oregon forms if they 

 are described as belonging to the previously described species, which 

 they most res,emble, treating under separate heads those that can not 

 be thus grouped. 



Ginkgo digitata (Brongniart) Heer. 

 PI. XXX, Figs. 1-7. 



1829. SpJuie.nopteris latifolia Phill. [non Brongn.] '' Geology of Yorkshire, pp. 148, 



189, pi. vii, fig. 18. 



1830. Cydo'pteris digitata. Brongn.: Hist. Veg. Foss., Vol. I, p. 219, pi. Ixi bis, 



figs. 2, 3. 

 1836. Adiantites digitatus (Brongn.) Gopp. : Sj^st. Fil. Foss., p. 217. 

 1843. Baiera digitata (Brongn.) Fr. Br. in Mimster: Beitr. z. Petrefacten-Kunde 



Vol. II, Heft VI, p. 21. 

 1865. Cyclopteris incisa Eichw.: Lethsea Rossica, Vol. II, p. 13, pi. iv, fig. 6. 



" The maidenhair tree, by A. C. Seward and Miss J. Gowan : Annals of Botany, Vol. XIV No LIII March 

 1900, pp. 109-1.54, pi. viii-x. 



'' Brongniart's Carboniferous species S-pherwfteris latifolia dates from the Prodrome (p. 51 ), one year earlier 

 than this, which is therefore to be dropped, although antedating all other names for this plant. — L. F. W. 



