122 THE POTOMAC OE YOUNGER MESOZOIC FLORA. 



teeth of varying depth, the basal ones subquadrate or ovate and spiny- 

 toothed ; the ultimate pinnae or pinnules gradually diminish towards the 

 ends of the compound pinnae ; the nervation in the pinnules and lobes 

 rather variable according to the position of the part, remote, very distinct 

 and strong ; the lov/er pinnules and lobes have a midnerve sending off on 

 each side alternate branches either forked or simple, in the upper ones 

 either once forked or simple. 



Localities: Fredericksburg; hill-side near Potomac Run; i-ed clay loall 

 in banks of Dutch Gap Canal ; rare. 



This handsome plant is characterized bv its subquadrate, acutely den- 

 tate lobes and pinnules, and by the remoteness and strength of the nerves 

 in them. It is not common at the localities where it is found. It is near 

 to Phillips's Sphcnopfcris qffinis. 



THYRSOPTERIS NERVOSA, Sp. nOV. 

 Plate XXV, Figs. 4, .^ Ifi ; Plate XXXVII. Figs. 2, 4; Plate XXXIX, Fig. 5; Plate XL, Fig. 6. 



Frond bipinnate or tripinnate, arborescent; principal rachis flexuous, 

 rather slender; ultimate pinnaj alternate, remote; pinnules alternate ; leaf- 

 suhstance thick, very obliquely inserted on the rachis, in shape oblong- 

 acute, in the lower ones cut rather deeply into subrhombic Ijasal lobes, 

 which toward the tips of the pinnules pass into ovate or elliptical lobes and 

 teeth, in the upper ones cut into elliptical acute lobes and teeth ; the pin- 

 nules narrowed at base nearly or quite into a petiole ; nerves very distinct, 

 slender, closely placed, going off from the midnerve very obliquel}^, diverg- 

 ing flabellately into each lobe and tooth, deeply forked, forking repeat- 

 edly, the branches subparallel. 



Localities : Fredericksburg ; hill-side near Potomac Run ; not very 

 rare. 



This is a very fine fern, and although the fragments found are not 

 large, they evidently belong to plants that must have attained very consid- 

 erable dimensions. It has some similarity to Thyrsopteris prisca Heer. 

 The fragment shown in PI. XXV, Fig. 16, has longer and narrower lobes 

 than the normal forms and may be different. 



