138 THE POTOMAC OK YOUNGER MESOZOIO FLORA. 



from one to the other as to forbid their separation. Plate LIT, Fig. 2, 

 shows no wing- on the rachis of the uUimate pinna, but shows a feature 

 connnon in tliis species, viz: the uhiniate pinnae on the upper side of the 

 peuuhimate rachis are hirger than those on the lower side. In Fig. 3, of 

 the same plate, the rachis is not winged, and the ultimate pinna3 do not 

 show the marked disparity in size often seen ; Fig. 4", of the same plate, 

 shows the ultimate pinntx- or pinnules decurrent, and forming a partial 

 wing to the rachis. The nerves in the lobes or pinnules of this form are 

 more numerous and deeply forked than in Fig. 2. In PI. LIII, Fig. 1, the 

 penultimate rachis is winged, and the pinnules, here reduced to lobe.s, 

 have remote, sparingly branched, strong nerves. The ultimate pinnaj do 

 not differ much in size. In Fig. 2, of tlie same plate, we have a surprising 

 difference in the size of the ultimate pinnae on opposite sides of the penulti- 

 mate rachis. In Fig. 3, of this plate, the usual disparity of the ultimate 

 pinna? is seen, and the pinnules differ from the normal ones markedly 

 in the nerves, which are fine, closel}' placed, deeply foi-ked, and com- 

 paratively numerous. In PI. LIV, Fig. 10, the basal upper pimiule is 

 decidedly longer than the rest, and directed upwards parallel with tlie 

 penultimate rachis, which is strongly winged. Tliis is probably the upper 

 portion of a compound pinna. This fern seems to be not closely allied to 

 any described fossil, and to have been a large plant. 



Thyrsopteris rhombifolia, sp. nov. 



Plate LII, Fig. 5; Plate LIV, Fig. 1. 



Frond apparently palmately divided; plant slender; principal rachises 

 slender, rounded, and prominent ; ultimate pinnie short, distant, o})posite, 

 pinnules remote; leaf-substance thick, upper basal pinnule larger than the 

 rest, more copiously and deeply lobed; pinnules subrhorabic to rhombic in 

 shape, slightly curved forward, acute, rarely obtuse ; nerves very distinct, 

 composed in the pinnules of a midnerve which sends off alternately and 

 very obliquely on each side forked or simple nerves, and which at its sum- 

 mit dissolves into branches. In the heteromorphous basal upper pinnule a 

 nerve-bundle goes off flabellately diverging into the basal lobes, the 

 branches of which are simple. 



