GEOLOCilCAL AFKlNlTlliS UF THE I'OTJ.MAC PLANTS 



343 



the Potomac, bein- one of the most characteristic pUxnts in it. It i-s sur- 

 pnsino- to ihul such a .lose resemWance between the most common iorms 

 of the^'otomac phuit and the Wernsdorf fossil. This species is more nnpor- 

 tant than a laro-e number of vaguely defined plants. 



TijsoHkr The trunks of Tysonia have their nearest relations ni later 

 Jurassic and Wealden forms, and hence may Nvell exist in a Neocomian 

 flora. 



CONIIT-KS. 



The conifers form much the most important and abundant element in 

 the Potomac flora. In development they surpass all other groups. The 

 proportion of conifers in the flora, taken alone, is a feature that strongly 

 indicates that it is Neocomian in age, for it seems that in this penod the 

 conifers, in proportion to the other elements of the flora, are more abundant 

 than they are at any other time. The Neocon.ian age of these plants is 

 indicated also by the near attiuity of man)- of the types to present forms, and 

 by the existence in it of very important an.l well-charactenzed genera that 

 .listingnish the Neocomian strata of various parts of the world. At the same 

 time however, there are in this flora a numljcr of uni.jue types, that indicate 

 that the Potomac was a period of rapid development for the conifers, m 

 which manv genera became extinct. The existence of these and a number 

 of -roups which might be regarded as prototypes or ancestral forms of 

 living genera shows that, notwithstanding the existence in it of a number 

 of an«>-iosperms, this is a very old flora. 



Xagdopsis: This remarkable genus is greatly developed in the Poto- 

 ,nac and highly characteristic of it. It is so much like the Nageia section of 

 Podocanms living in the East Indies, Java, and Japan, that it is dithcult to 

 resist the idea that there is a genetic connection between them; y<u,na 

 beino- a surviv.M- and descendant of Xaficiopsis. This is confirmed by the 

 remrrkable isolation and limited development of Nn,yut. It is interesting 

 to note the fact that so many of the characteristic coniferous types of the 

 Potomac flora have their nearest living representatives in certain genera ot 

 very limited development and restricted distribution confined to the eastern 

 coast of Asia and to Australia. Among these may be mentioned, in addi- 

 tion to NaQciopsis, .l/A>-otooj«^s, now represented by Athrota.as in las- 



