102 THE POTOMAC OE YOUNGER MESOZOIC FLOEA.. 



This very finely cut feni is found most abundantly at the fishing hut 

 above Dutch Gap Canal, but is not very common even there. The sori 

 are so small and the fertile pinnules so leathery that they can be seen 

 only when the lower surface is uppermost. In PI. XXII, Fig. d% is given 

 a magnified pinnule with its lower surface uppermost; in Fig. 9'' a similar 

 pinnule with its upper surface uppermost, and showing no sign of sori. 

 This latter is the common aspect of the plant. This may explain why the 

 sori were not seen by Dunker and Schenk. It seems to be certainly 

 Dunker's species. The lobes in most of the specimens seem to be moi-e 

 closely placed than in Dunker's plant. It is one of the most widely 

 diffused ferns of the Potomac flora. 



ASPIDIUM DENTATUM, Sp. UOV. 

 Plate XXV, Figs. 6, 7, 14, 15. 



Frond tripinnate ; principal rachis comparatively stout and rigid; ulti- 

 mate pinnge alternate, very short, linear- lanceolate ; pinnules membrana- 

 ceous, alternate, lower ones separate to the base, upper ones united and 

 reduced to lobes; lower pinnules broadly ovate, acute, narrowed to the 

 base, and subpetiolate, cut nearly to the midnerve into ovate, acute teeth, 

 reduced in the upper part of the compound pinuiE to ovate, obtuse lobes 

 and teeth ; sori verj- small, subreniform or globose, placed within the mar- 

 gin on the summit of the alternate simple lateral nerves ; nerves in each 

 of the pinnules and lobes, composed of a midnerve, with simple alternate 

 lateral ones on each side ; in the uppermost lobes the nerves become one 

 or more times forked, without a midnerve. 



Localities: Red clay ball in banks of Dutch Gap Canal; hill-side near 

 Potomac Run; near Telegraph Station; rare in most of the localities. 



This pretty little plant is rather abundant at the locality on the hill- 

 side near Potomac Run. Forms such as Fig. 15 are quite common here, 

 but as a rule show no fructification. Tlie fructified specimen. Fig. 14, was 

 found in the red clay ball in the banks of Dutch Gap Canal. Fig. 15 is a 

 good deal like the smaller forms of Thyrsoptcris Murrayana (Brongn. sp.) 

 Heer, but is no doubt a distinct species. 



