5t;8 MESOZOIC FLORAS OF LNirED STATES. 



could not 1)P taken as iiulicating that the phmt was common in the flora. 

 Nayciopsis angustifolia, Thyrsopteris Meckiaiia anguslilobu, T. ntrinervis, 

 Cla(loj)lilcins Bi'owniana, Leptostrolnis lorigifolins, BrachyphyUinn cntssi- 

 caulc, and Cladophlehis aUitd, in tlie Mi'ginia beds, pass from the liappa- 

 liaiiiiock into the Aquia Creek series, all except the first being mvich more 

 characteristic of the Rappahannock than of the Aciuia Creek horizon. 

 Xngciopsis angustifoUa is probal)Iy more common in the upper than in the 

 lower beds. But the small number of these species impairs their value as 

 determiners of age. Nageiopsis angustifolia is the only one whose 

 abundance entitles it to much weight, and it is almost as abundant in the 

 lower as in the upper member. The plants that are confined to the 

 Rappahannock series are these: 



Acrostichopteris parvifolia. 

 Nageiopsis heterophjila. 

 Thyrsopteris pachyrachis. 



Here we have the plants more abundantly represented in the Balti- 

 more beds, and this is especially true of Acrostichopteris i>arvifolia, with 

 19 specimens. The genus Acrostichopteris is of more value for determin- 

 ing the age of these beds than any other of those named above. In the 

 first place we may note that all the occurrences of the species of this 

 gemis, other than those on the horizon of the Baltimore beds, are confined 

 to the Rappahamiock portion of the Lower Potomac. The particular 

 species now in question is found in the localities on James River men- 

 tioned in Monograph XV, rarely at the entrance to Trents Reach, and 

 rather abundantly at the fishing hut above Dutch Gap Canal. 



By far the most important plants of the Baltimore beds are not 

 found in the Virginia Potomac, and can not help the comparison. It 

 is a noteworthy fact that so many species occur in the Baltimore strata 

 that are peculiar to them, and that some of these show so many speci- 

 mens here while they are wholly wanting on other horizons. It might 

 be taken to mean that the Baltimore horizon is wanting in Virginia and 

 the District of Columbia, where also they are not found. It is more 

 ])robat)le, however, that it is explained by the very local distribution 

 of the elements of the Lowei' Potomac flora. This is indicated by the 

 local distribution of many forms in the Virginia Potomac. 



