334 THE POTOMA.C OR YOUNGER MESOZOIC FLOEA. 



fixing age as species attaining their maximum development in the age in 

 question. 



One of the most noteworthy features of the Potomac flora is the over- 

 whehiiing predominance of new species and the proportionally very large 

 number of peculiar types. It is true that, owing to the absence of the flow- 

 ers and fruit, we may in many cases fail to place with accuracy the forms 

 in still living groups ; but still, in a ver}^ large number of cases, enough is 

 shown in the foliage to indicate that it differs in type from that of the plants 

 of other formations, and in a considerable number of instances the charac- 

 ter is so remarkable and well defined as not to require any other evidence 

 to prove the uniqueness of the plants possessing it. 



We might explain the remarkable abundance of new forms in the Po- 

 tomac by assuming, for the region and period possessing this flora, unusual 

 topographical or climatal features, but this is rendered improbable by the 

 fact that the flora of the formation possesses a number of species known 

 from other parts of the world. When we consider the distance that sepa- 

 rates the countries in which these common species grew their similarity is 

 surprising. 



It is more probable that the peculiarity of the Potomac flora is to be 

 explained by the fact that it belongs to a period whose land plants have not 

 been found elsewhere, or if found are poorly represented. As will be seen 

 later, this conclusion tallies with the deductions to be drawn from an exami- 

 nation of the individual elements composing the flora. If, as we are led to 

 think from a study of the plants composing this flora, its age is Neocomian, 

 then we should expect to find in any large collection of its fossils a great 

 proportion of new forms ; for the Neocomian, taken the world over, is pre- 

 dominantly marine, and its land flora is one of the least known. 



Taking up now the diff"erent groups composing the Potomac flora, we 

 find the Equiseta first in order. 



EQUISETA. 



In passing from the flora of the Rhsetic beds of North Carolina and 

 Virginia to that of the Potomac, we find a very marked change in the 

 Equisetacece. There is in the Potomac no trace of Schisonenra, a group 

 meagerly represented in the Rhtetic flora in foliage at least. 



