OLDER POTOMAC OF VIRGINIA AND MARYLAND. 567 



only will be omitted. The plants that are available for this comparison 

 are the following : 



Acrostichopteris parvifolia, with 19 specimens. 



Brachyphyllum crassicaule, with 3 specimens. 



Cladophlebis alata (Pecopteris stricfcinervis type) with 1 specimen. 



Cladophlebis Browniana, with 2 specimens. 



Leptostrobus longifoHus, with 2 specimens. 



Menispermites tenuiaervis, with 4 specimens. 



Menispermites vu-giniensis, with 1 specimen. 



Nageiopsis angustifoha, with 8 specimens. 



Nageiopsis heterophylla, with 4 specimens. 



Thyrsopteris Meekiana angustiloba, with 1 specimen. 



Thyrsopteris pachyrachis, with 5 specimens. 



Tliyrsopteris rarinervis, with 1 specimen. 



Ament of conifer (b), with 1 specimen. 



Menispermites virginiensis, M. tenuinervis, and ament of conifer (b) 

 are the only plants in this list that have not been found in the Rappahan- 

 nock series or the lower portion of the Lower Potomac of Virginia. As 

 only one specimen of the first of these occurs it can not have much impor- 

 tance in the flora and might be interpreted as an anticipation of a later 

 flora, and not j^et fully established, for it is more at home in the Aquia 

 Creek horizon. But Menis'permites tenuinervis, from the number (4) of 

 its specimens, is of more importance. This is an important form in the 

 collections from Fort Foote, which is on the Aquia Creek horizon, and it 

 is abundant in the Mount Vernon group of beds. If, guided by the general 

 difference in the flora, we divide the Lower Potomac into two horizons, 

 the lower would contain the James River and Rappahannock series of 

 Professor Ward, with essentially the same flora. The upper would 

 embrace the Aquia Creek or Brooke beds and the Mount Vernon group, 

 the latter being somewhat the older of the two. The upper member, as 

 before stated, differs from the lower in having a decidedly smaller pro- 

 portion of survivors of the Jurassic flora, few of the more archaic dicoty- 

 ledons, and a larger proportion of dicotyledons of modern aspect. So far, 

 then, as the evidence of Menispermites tenuinervis goes, the Baltimore 

 beds would be placed on the Aquia Creek horizon. But even in this case, 

 only four specimens being found in collections embracing hundreds, they 



