174 MESOZOIC FLORAS OF UNITED STATES. 



of their determination. To have vahie in determining age the mere 

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

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

 an older 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 Saportana, Ginkgo digitata, 

 G. Huttoni magnifolial, and Baiera gracilis are notabty fossils of the 

 Lower Oolite. Ginkgodiumi 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 Ci'etaceous, 

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

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

 folius"], Nageiopsis 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 the^^ were not 

 abundant in the flora, and they may be survivors from an older one. 

 Podozamites grandifolius% and Ginkgo Huttoni magnifolia'^ 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 alata, Onychiopsis psilotoides, Cladophlebis 

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



The Cladophlebis psilotoides shows some differences 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 number of their specimens, must have been 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 overwhelming 

 in favor of the Lower Cretaceous. 



I am, however, 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, 



