526 



MESOZOIC FLORAS OF UNITED STATES. 



of Virginia, and not that of the Brooke or Aquia Creek strata. A brief 

 resume may make this plainer. For the sake of reference in this account 

 I will refer to the basal part as the Rappahannock beds. The following 

 plants, being doubtfully determined, will not have so much weight in 

 determining age: 



Quercophyllum tenuinerve 1 

 Rogersia angustifolia ? 

 Thyrsopteris divaricata? 



Ctenopteris insignis ? 

 Ficophyllum tenuinerve ? 

 Nageiopsis heterophylla ? 

 Platypterygium densinerve ? 



All of these are confined to the horizon of the Rappahannock beds. 



We may also omit the new variety, Rogersia ayigustifolia parva, as 

 it does not occur in the Virginia beds. 



This leaves, as having considerable value in determining the age of 

 the Langdon beds, the following species: 



Sphenolepidium Sternbergianum densi- 



folium. 

 Thyrsopteris decurrens. 

 Thyrsopteris insignis. 

 Thyrsopteris nervosa. 

 Thyrsopteris rarinervis. 

 Zamiopsis insignis. 



Athrotaxopsis expansa. 

 Athrotaxopsis tenuicaulis. 

 Cycadeospermum ellipticum. 

 Cycadeospermum obovatum. 

 Glyptostrobus (Taxodium) brookensis. 

 Nageiopsis zamioides. 

 Rogersia longifolia. 

 SaHciphylluni ellipticum. 



The two species of Athrotaxopsis and the two of Cycadeospermum, 

 Nageiopsis zamioides, Thyrsopteris insignis, and T. nervosa are all con- 

 fined to the Rappahannock horizon. Rogersia longifolia, Saliciphyllum 

 ellipticum, Sphenolepidium Sternhergianum densifolium, Thyrsopteris 

 decurrens, and T. rarinervis, though found in the lower beds, occur also 

 on the Aquia Creek horizon. Glyptostrobus (Taxiodium) brookensis is 

 the only plant confined to the horizon of the Aquia Creek beds in Virginia. 



The absence of all dicotyledons of the more modern aspect and the 

 presence of only a very few of the archaic type, characteristic of the 

 basal beds, is strong proof that the strata are older than the Aquia Creek 

 beds. It is safe to affirm that the latter will, in all cases, contain a large 

 element of dicotyledons of comparatively modern aspect. 



