DESCRIPTION OF THE SPECIES. 105 



of a lens over the surface of tlie soi-i, as if caused Ijy tlio sporangia. No 

 indusiuni appears. 



Tiiis fossil is a good deal like the living FfKlifoiia proViferd, and the 

 .specific name is chosen from tliis resend)lance. It is a little like Natlioistia 

 anyustifulia Ileer,' hut is distinct from that plant. 



PoLYI'ODlUM DKNTATUM, sp. HOV. 

 Plate XXII, Figs. 1, r.. 



Frond bipinnatitid; piiuia^ in the sterile forms of ultimate order, or 

 pinnules linear-elongate, cut more or less deeply into broad, ovate, slightly 

 falcate, acute lobes or pinnules; midnerve of the pinn;v strong and rigid; 

 nerves of the lobes or piinuv composed of a nerve-bundle formed b}' a 

 pai'ent nerve which goes ott' obliquely and curves outward towards the 

 outer edge of the lobes, sending oH branches onl}' from the upper side; 

 the branches curved strongl}* outward, and each twice forked, except the 

 upper one, which is once forked; fertile pinna- or pinnules serrate, 

 narrowlv linear; nerve-bnndles in each tooth branching and diverging 

 flabellately from the insertion, the lower branch forking several times, the 

 upper one simple and directed obliquely upwards, bearing on its sunnnit 

 in each tooth an obovate sorus. 



Locality: Fredericksltui-g; very rare. 



Thi.s seems to be a very distinct species. The lens sjiows a granula- 

 tion over the surface of the club-shaped sori which seems to be cau.sed by 



tlio sporangia. 



ACROSTICIIUM, L. 



Tiie genus ArrosfirJiuin seems to be represented in the Potomac flora 

 by the small fragment described below. It also has near allies in the new 

 genus Acrostichopteris, so abundant at H.dtimore. 



AcRCSTicnuM CRASsiKoi.iuM, sp. nov. 



Pliite XVI, l-'ig. 7. 



Frond and pinufc not seen ; pinnules very obtuse, thick and leathery 

 in texture; nerves not seen distinctly; sori small, in three double rows, 

 each row separated from its fellow by a pretty distinct line, which is 



'Flor. Fobs. Arc, vol. C, part 1, No. 2, PI. I, Figs. 1-6. 



