DESCRIPTION OF THE SPECIES. 89 



fonu :i winy, w'lilx margins iiiidulatu cir creiiatf; leaf-substaiR'e very thick 

 and leathery; nerves composed in each pinnule of a midrib, which extends 

 to the tip, and sends oflf alternately on each side into tlie teetli simple 

 branches, all shar[)ly defined. 



Locality: Fredericksburg; not unconnnon. 



This little fern, although found only in small fragments, seems to be 

 very well characterized. It has some of the features of a Thyrsoptcris, and 

 perhaps should be placed in that genus rather than in Pecopteris. It is not 

 closely allied to any described plant known to me. 



SPHENOPTERIS, lirongn. 



Herbaceous plants, with fronds ranging from pinnate to tripinnatifid; 

 pinnules cuneate or lobed, lobes dentate or subdivided ; ])rimary nerve 

 slender, towards the summit often bifid or dissolved into brandies ; second- 

 ary nerves diverging towards their extremities, or prolonged into the lobes 

 or teeth. 



This genus is another of the heterogeneous groups which serve to col- 

 lect together ferns w Inch may belong to quite distinct genera and species, 

 but which have in common certain features not dependent upon the fructi- 

 fication, and are not otherwise characterized. The number of ferns of the 

 SpliPnoptvris type found in the Potomac flora is snuill. T phice provision- 

 ally in this genus a small num])cr of forms. Tiicn^ are a inimber of jdants 

 in the Potomac flora, especially those of the t3-i)e of Spltaioiitcris MantclU 

 Brongn., wliich have predominantly the character of Thtjrsopteris. These I 

 have placed under this latter genus. 



Sphenoptkkis thvrsopteroidks, sp. nov. 



PliitoXXV, Fiij. :{; Plate I.VIII, Fi^. .">. 



Frond unknown ; rachis of the principal jtinn;!; winged, ultimate pin- 

 na3 opposite or alternate, sliort, and terminating in incised and lol)ed seg- 

 ments with acute tips, broadly winged; pinnules narrowed at the base, 

 incised into oblong acutely dentate lobes, or cut into oblong acute teeth; 

 leaf-substance thin ; nerves few and distinct; nerves in each pinnule com- 



