338 THE roTOMAC ok youxoer mesozoig flora. 



matters now stand, Sphenopterls MaiitdU is the name of a particular 

 type of foliage and nervation very common in Wealden ferns rather than 

 the name of a single species. But liowever this may be, the evidence 

 given by the presence of this type of fern is not affected. The existence 

 in a flora of a large element in the ferns which has the type of S. MuiiteUi 

 may be taken as indicating affinities with the Wealden flora. 



Now in the Potomac flora not only is -S". ManteUi present in beds 

 which show plants of the most recent facies existing in the formation, but 

 there is a very important group of ferns which, although placed in the 

 genus Thi/rsopteris, have nearly the nervation and foliage typified in S. 

 ManteUi. The great development in the Potomac of ferns of the general 

 type of S. ManteUi gives strong evidence of Wealden or somewhat later 

 age. A somewhat later age than Wealden is indicated, perhaps, as most of 

 the species are somewhat modified, so as to depart more or less from the 

 typical S. ManteUi, and to assume the facies of Thyrsopteris. The other 

 species of Sphenopteris give little help in fixing the age of the Potomac 

 strata. 



Aapidhuit: The genus AspidiumvfonlA seem to be, when largely devel- 

 oped in a flora, a decidedly recent element. The finding of as many as 

 fourteen species of true Aspidium in the Potomac flora, and the compara- 

 tively large number of individuals shown by some of these, would, if taken 

 alone, indicate a very recent age for this flora. There is nothing in the 

 foliation or in the fructification to make us think that these plants could not 

 be still living. Aspidium Oerstedi is determined as present, but the identi- 

 fication is doubtful, as the amount of material found is too small to permit 

 any sure conclusion. With the exception of A. Dankeri Schimper, the other 

 species are peculiar to the Potomac formation. Pecopteris Dunkcri Schim- 

 per, of the Wealden of Europe, seems to be quite characteristic of that for- 

 mation. It is pretty certainly an Aspidium, as shown by the Potomac 

 forms. It is a plant of considerable importance in establishing the exist- 

 ence of Wealden elements in the Potomac flora. 



Polypodium and AcrosticJmm : These two living genera are probably 

 sparingly represented in the Potomac flora. Of coui-se, as far as their evi- 

 dence goes in this flora, they establish relations with living plants rather than 



