GEOLOGICAL AFFINITIES OF THE I'OTO>[AC PLANTS. 347 



The cliaracter of the Potomac angiosperms shows tliat the first advent 

 of this chiss must be looked for still further l)ack in time, but the Potomac 

 flora gives us the first appearance of this high type in an}- notal)le amounts, 

 and shows that these plants, at least as early as the oldest Cretaceous, were 

 present in considerable numbers, thus pushing back by at least one epoch 

 the date of their considerable development. It should be noted that we do 

 not find in them, although many of them are (piite archaic in type, any 

 forms establishing a transition between angiosperms and the ferns or gym- 

 nosperms. 



The examination of most of the genera in this group will give us l)ut 

 little help in making out the precise age of the Potomac flora. ^lost of 

 them are too poorly characterized or too remote from living genera to 

 make a coniparative study worth much. Tin; fact, however, that a number 

 of the most peculiar forms seem to be generalized t)'pes, indicates a very 

 considerable antiquity for the flora. 



As, however, all or very nearly all of the species of these angiosperms 

 jiassed away before the advent of the flora of the New Jersey Cretaceous, 

 we are entitled to conclude even from this group that the Potomac flora is 

 older than the Cenomanian. Such genera as Acackephyllum, Frofecq^hyllum, 

 and lioyctsia seem to be very archaic. The same may be said, perhaps, 

 of CrhistropJii/llum, and certainly of Fkophyllum. ropulophijlhun, Qucrco- 

 plif/Iliiin, UhniphijUnm, Saliciphi/Uum, Juglandiphyllum, and Eucalyptophylhim, 

 etc., are too imperfectly characterized to be of use. 



Such genera as Sassafras, Vitiphyllum, Sapindopsis, Myrica, StcrcuUa, 

 Hymoicca, and Mcnispermites are found in the Cenomanian or have closely 

 allied forms in it. VitiphyUum is very near Cissitcs, and Sapindfpsis is very 

 close to Sapindus, both being genera occurring in the Cenomanian. All 

 these, however, are represented in that formation by species different from 

 those of the Potomac. 



The remarkable leaves shown in AraUdphyllum, AcGripIiyllitiii, and JIc- 

 dcnrphyllum, as illustrated in H. anyulatiim, seem to be comprehensive types, 

 and hence are probably old forms 



Taken as a whole, then, and compared with the Cenomanian flora 

 of the Dakota and New Jersey Cretaceous strata, the angiosperms of 



