GEOLOGICAL AFFINITIES OF THE POTOMAC PLANTS. 345 



Urgonian trroups. Frcnclnjhsis ramo.<^issi))ia is so imicli like Froicia tliat a 

 genetic connection can hardly be denied, and tliis latter would sccni to lie 

 a younger form than F. imrcerumosa. 



Brachyplnjlhu)! : The Potomac forms of lirachf/phi/llitm are most like 

 Heer's BrncJiDpJn/lhnn ohcsiim from the Wealden of Portugal. 



Leptostrohus : This is Jurassic, and lends additional weight to the (»lder 

 elements of the flora. 



Laricopsis : This must be regarded as a recent element, but it differs 

 from Larix enough to render it doubtful whether it could be regarded as 

 indicating any particular post- Jurassic time 



Tonei/a: This belongs to the more recent forms, but it is too slightly 

 developed in the flora to have much weight. 



Cephalotaxopsis : Thjs is a very" important and highly characteristic 

 plant. It is unmistakable in character, widely diffused, and represented by 

 several species, some of which seem to have been abundant. It is so much 

 like Cephalotaxits that possibly it should not be separated from it. It adds 

 niucli weight to the more recent elements of the flora. 



Athrofaxopsis : This is very near the living genus Jllirolaris, and the 

 same may be said of its significance that was said of Crphalotuxnpsis. 



Srquoin: "^Phis is the most important genus of conifeis in the Potomac 

 formation, on account of both its large development and the nuniVier of 

 species that it shows common to the Potomac and to the lower Cretaceous 

 tlonis of other parts of the world. Thus we find in the Potomac and in 

 the Urgonian strata of Koine, Greenland, the species -S'. Iirichnilxiclii, S. 

 (imhlffwi, S. nf/i(hi, S. grnrUis ; and, in addition, the species S. sithididn is 

 found in both tlm Potomac and the Atane, or middle Cretaceous, beds of 

 (Jreenland. Considering the number of species, this is proportionall}- a 

 larger number of conmion ones. Sct/noin gives more previousU' known 

 species than any other genus of plants. 



AiaKCKtid: This genus l)egins in the .Turassic and comes down to the 

 present time, so that the Potomac species, being peculiar to it, can not 

 indicate any i)articnliir j)ost-Jurassic era. 



Tax odium {(ihiptostrohus): The Cilyptostrobus t}pe of 'I'axni/iiim is 

 most characteristic of the middle and upper Cretaceous ; and the Potomac 



