342 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



in the fossil floras of countries now possessing a cool temperate climate. 

 We are, then, from a consideration of the family as a whole, entitled to 

 hold that the Potomac flora can not be later than Neocomian in age. The 

 character and the degree of development of the particular genera and 

 species point unmistakably to the Neocomian as the age of this formation. 



Anomozamites : This is an old type, belonging to the Rluvtic and older 

 Jurassic. Anomozamites proper is but slightly developed in the flora, and 

 at the same time, not being clearly established by the few and imperfect 

 specimens found, may be i^assed by. The case is different, however, with 

 the subgenus Platypterigium. This is a well-characterized form, and the 

 comparatively large number of fairly well preserved specimens found at 

 Fredericksburg give it considerable weight. The genus seems to be most 

 characteristic of the Rhaetic and older Jurassic, but it almost certainly 

 survives until the middle Cretaceous in Heer's Ntlssonia Johnsiriqn, from 

 the Atane beds of Greenland. There is, then, nothing strange in finding 

 this genus in Neocomian strata. 



Zamites: This is eminently a later Jurassic genus, but the Potomac 

 specimens are too imperfect and uncertain to be used. 



Encephalartoiisis : This genus, from its nearness to the living Encepha- 

 lartos, is a recent element in the flora. From its great rarity, however, it 

 is not entitled to much weight, and must be regarded as a newly introduced 

 form. 



Ctcnophyllum : This is an old type. As it is exceedingly i-are and 

 dififers a good deal from the most common Rhaetic form, it must be regarded 

 as a survivor. 



Ctcnis: The same may be said of this as of CtenophijUum. It is a 

 considerably modified survivor from the Rha»tic and Jurassic floras. 



Podozamites : This has no value in fixing definitely the age of a flora, 

 for, as the name has been used, it ranges throughout the Mesozoic. In 

 addition to this, the Potomac yields only detached and sparingly dispersed 

 leaves. 



Dioonites: This genus, known from the Neocomian (Urgonian) of the 

 Wernsdorf strata, is entitled to great weight in fixing the age of the Poto- 

 mac. It is well characterized, widely distributed, and very abundant in 



