110 THE POTOMAC OE YOUNGER MESOZOIC FLORA. 



on each side of the midnerve, and not extending to the tips of the pin- 

 nules. 



Locahty: Fredericksburg; only one specimen found. 



This plant is in too fragmentary a condition to make out its true char- 

 acter. The shape of the pinnules is something like that of those of Bun- 

 ker's Neuropferis Albertsii. The plant is (piite near to Asplenium argutulum 

 Heer.^ 



THINNFELDIA Ett. {ex parte). 



Fronds bipinnate or tripinnate ; pinnules varying much in size and 

 shape, mostly oblong, ovate-lanceolate, or oblong-obovate, decurrent and 

 mostly confluent at base, coriaceous; primarv nerve of the pinnules dis- 

 solved before attaining the apex into many dichotomous nerves ; secondary 

 nerves going off at a very acute angle, diverging in ascending, several 

 times dichotomous. 



The description given above for the genus is very nearly that given 

 by Schimper. There are several species of ferns in the Potomac flora 

 which would come under the group having tliese characters. The plants, 

 however, are.not common, except at the 72d mile-post, near Brooke, where 

 Thinnfeldia variahills is the most common fern. 



Thinnfeldia VARiAnrLis, sp. nov. 



Plate XVII, Figs. 3-7; Plate XVIII, Figs. 1-6. 



Frond bipinnate or tripinnate ; rachises of the pinnje stout, arbores- 

 cent ; pinnules lobed or toothed, with mostly ovate-obtuse lobes or teeth, 

 short, varying much in size and nature according to position on the frond, 

 passing in ascending, in the upper part of the pinna}, through toothed and 

 undulate pinnules into entire ones ; lower pinnules attached by the rachis 

 alone, and deeply cut into broadly ovate-obtuse lobes ; up higher in the 

 j)iunaj and on the frond they have ovate teeth and are attached by the 

 middle of the nuich-narrowed base, and are slightly decurrent ; towards 

 the top the ultimate pinnae pass into crenately lobed segments and these 

 into lobed and toothed pinnules, like those lower down ; all are acute ; 



iFlor. Foss, Arc, vol. 4, pt. 2, Beitriige zur Flor. Ostsib., PI. Ill, Fig. 7 ; PI. XIX, Figs. 1-4. 



