174 :\IESOZOI(;' FLORAS OF UNITED STATES. 



of their (lotorniination. To have value in determining age the mere 

 presence of the species is not all tliat is required. The plant must be 

 abundant in the flora and characteristic of it. It may be a survivor from 

 an oldei- flora. The proportion of specimens in a collection is the only 

 feature that gives a hint on these points. It of course must not be insisted 

 on too strongly, for there are other conditions besides the actual relative 

 abundance that may give a large proportion of the specimens to one 

 species. Cladophlebis vaccensis, Dicksonia Saixirknia, Ginkgo digitata, 

 G. Huttoni rnagnifolia'^., and Baiera gracilis are notably fossils of the 

 Lower Oolite. Ginkgodinml alaskense is a new species, and if it be a 

 true Ginkgodium, its nearest kin is found only in the same formation. 



On the other hand, the following belong to the Lower Cretaceous, 

 taking the Wealden as belonging to that formation: Cladophlebis alata, 

 C. Huttoni, Onychiopsis psilotoides, Podozamites distantinervis, P. grandi- 

 folius'!, Xageio))sis longifolia. 



These plants are, as stated, not of equal value in determining age. 

 Cladophlebis vaccensis, Dicksonia Saportana, and Baiera gracilis have 

 each only one specimen. Hence we may conclude that they were not 

 abundant in the flora, and they may be sui'vi\'ors from an older one. 

 Podozamites grandif alius"] and Ginkgo Huttoni magnifoliai are not posi- 

 tively determined. Leaving these and the probable Ginkgodium out of 

 the question, we have, as the fossils of most value for fixing the age, five 

 plants, viz, Cladophlebis (data, Onychiopsis psilotoides, Cladophlebis 

 Huttoni Podozamites distantinervis. Ginkgo digitata, a very small list. 



The Cladophlebis psilotoides shows some difi"erences from most 

 of the described Lower Cretaceous forms, which somewhat impair its 

 value as evidence. Cladophlebis alata and C. Huttoni, if we may judge 

 from the mimber of their specimens, must have l)een abundant and highly 

 characteristic of the Alaskan flora of their time. All of the more im- 

 portant plants except Ginkgo digitata are Lower Cretaceous, and if we 

 take simply their percentage in the flora the evidence is ovei-whelming 

 in favor of the Lower Cretaceous. 



I am, ho^\•e^'er, inclined to attach great weight to the considerable 

 proportion of Ginkgos of Jurassic type. 



The Ginkgos in Lower Oolitic times were immensely developed in the 

 Amur region in Siberia and in the northwestern part of the United States, 



