GEOLOGICAL AI'FINITIES OF THE POTO.MAG PLANTS. 341 



ht'iiig- fully ilirt'ereiitiated. If so, this g-enus can not be used in (U-terinininj^ 

 the age of the strata. It would simply indicate for them a very consider- 

 able anti(inity. 



Sagenoptiris : This is a well-marked genus, not easily mistaken for any 

 other, and hence it is valuable in determining age. It has such a wide range 

 in time, however, that it will not enable us to fix within narrow limits the 

 horizon of the beds containing it. It appears to extend from the l\h;vtic 

 into the lower Cretaceous or Neocomian, surviving in the Xeocomian of 

 Westphalia. 



Sderoptcris : This genus is well characterized, so that it is readily 

 detected. It has a considei-able number of species in the Potomac beds, 

 although none of them are anywhere connnon. This group of plants 

 gives to the Potomac flora one of its most important Jurassic elements, tor 

 this type is eminently characteristic of the later Jura.ssic. It must be 

 regarded as a survivor in the Potomac, coming down from an older flora. 



Ctenopteris : This genus has, so far as the question of age is concerned, 

 \ ery nearly the same significance as Sdcropteris, being Jurassic. liut the 

 Ctenopterids of the Potomac appear to be modified descendants of the 

 forms found in the older floras. They do not necessarily imply an age as 

 great as even that of the Oolite, and it should be noted that some of them 

 much resemble Ileer's genus Ctcnidhuu, found in the Wealden of Portugal. 

 There is nothing in the general character of these Potomac plants that 

 would make it improbable that they occur in the older Neocomian. 



Zamiopsis: This is a remarkable type, peculiar, so far as known, to 

 the Potomac. It can not help us in determining the question of age. 

 This genus appears in some points to be allied with Ctoio/iferiv, for it has 

 the same curious union of the charactei's of ferns and cycads. 



The comparatively large number of cycads in the Potomac strata 

 does much to separate the flora of this formation from that of the Oeno- 

 manian, and to increase in it the resemblance to the tyjjical Mesozoic or 

 Jurassic flora. Tliey indicate that in the Potomac tropical conditions still 

 prevailed to a considerable extent. Cycads are rare after the Neocomian 



