94 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



development of this genus introduces a strong modern element into the 

 flora, and it is a noteworthy feature that so many well-characterized species 

 of this genus occur with numerous forms belonging in type to the Jurassic 

 flora. 



The tribe of the Aspidiese is represented in the Potomac flora by 

 forms that belong to the generic tj^pes of both Aspklium proper and 

 Didijmochlcena. As with these imperfect specimens the identification can 

 not be certainly made with the one genus or the other, it seems best to 

 place them all provisionally under the principal genus Aspidimn. It 

 should be noted, however, that some of these species may belong not to 

 the tribe of Aspkliem, but to that of the DavaUlece. 



AspiDiuM Feedericksburgense, sp. nov. 



Plate XI, Figs. 1-6 ; Plate XII, Figs. 1-G ; Plate XVI, Fig. 9 ; Plate XIX, Figs. 6, 7. 



Frond bipinnate or tripinnate, arborescent; rachis of the penultimate 

 pinnse very stout and rigid ; pinnse of the ultimate order mostly alternate, 

 rarely opj^osite or subopposite, with rigid and proportionally rather slen- 

 der i-achises, very long, linear ; pinnules alternate, oblong or ovate, obtuse, 

 slightly falcate, and usually somewhat rounded and narrowed at the 

 attachment to the rachis, separate, those of the lower pinnse with crenate 

 margins, those of the upper ones entire, passing in the middle part of 

 the frond through piimules with undulate margins; leaf-substance thick 

 and leathery ; midnerve similar to that of ('ladophlehis, that is, strong at 

 base and dissolving into branches at the summit ; lateral nerves of the cre- 

 nate and undulate pinnules in groups in each tooth, composed of a mid- 

 nerve which sends off alternate simple branches, or else of forked nerves 

 with one of the branches forking again ; those of the pinnules with 

 entire margins usually once forked, all quite distinct; sori very large, reni- 

 forni in shape, and distributed in two rows, one on each side of the mid- 

 nerve, attached to the suuuuit of the upper branch of a furcate nerve. 



Localities: Fredericksburg; and near Telegraph Station. 



This plant is one of the most common ferns at Fredericksburg. All 

 the specimens figured come from this pla,ce, except that in PI. XI, Fig. 1, 

 which comes from near Telegi'aph Station, where the plant is not rare. 



