336 THE POTOMAC OR TOUNGEE MESOZOIC FLORA. 



conifers, and equiseta. In such a flora generally the ferns and conifers 

 stand first in the number of kinds and of individuals, the cycads usually 

 coming next, and the equiseta last. Were it not for the considerable 

 angiosperm element in the Potomac flora it would be typical Mesozoic, and 

 this would be largely due to the great variety and the universal diffusion 

 of the ferns. While the number of different kinds of ferns is proportionally 

 very large, the number of individuals of each kind is, comparatively 

 speaking, very small. There is also a curious mingling of older and more 

 recent types, and a remarkable localization of many of the species. The 

 species identified with those of other floras seem to be mainly a few world- 

 wide and long-lived forms. Many of the older types seem to be survivors 

 from the Jurassic. The flora of the Cenomanian is quite well known from 

 a number of widely separated regions. The fact that nearly all of the 

 ferns of the Potomac are absent from the different Cenomanian floras is 

 strong evidence that the Potomac flora is older than Cenomanian. 



Let us now rapidly examine the evidence to be deduced from the dif- 

 ferent groujjs of ferns. CladophJehis : The ferns placed in the genus Cla- 

 dophlehis do not, it is true, show fructification, but the plan of their foliage 

 and nervation is a well-marked one, which is, the world over, very charac- 

 teristic of Jurassic floras. The great number of species of this type found 

 in the Potomac flora is certainly significant. In no flora which does not 

 have strong Jurassic affinities could such a large proportion of the ferns be 

 of this type. It would seem that the conditions prevailing in Jurassic 

 times favored the development of this particular kind of foliage, and we 

 may assume that in the Potomac era these conditions had not wholly passed 

 away. 



The presence of such a large element, having the facies of Cladoplilebis, 

 is entitled to more weight in fixing the geological relations of a flora than 

 the establishing of the identity or near relationship of a few species in 

 two widely separated floras. But we are not without confirmatory evi- 

 dence derived from this source. The species of this genus in the Potomac 

 flora are probably all new, but some of them differ so little from some of 

 those occurring in the Rhsetic and Jurassic floras, that they may fairly be 

 taken as survivors and representatives of the older species. For example, 



