DESCRIl'TION OF TOE Sl'EClKS. 119 



This small plaur was t'oiunl in too fragmentary a coiKlition to permit 

 its full character to he made out. It lias some features that resemble those 

 of Adiantum, especially the terminal position of the elongate sori on the 

 basal upper lobes and the two terminal segments. I'he basal upper seg- 

 ment is much larger than the others, and, unlike them, is subdivided into 

 several segments, wliich bear the sori on the tips of the lobes. 



GLEICHENIA Swartz. 



Tlie genus (Hcichenia seems, if present at all in the Potomac Hoi-a, to 

 be very feebly represented. In this respect the Potomac Hora di tiers 

 greatly from that of nearly the same age described by Ileer as coming 

 from Kome, Greenland. 



Gleichknia Nor])Enski(')Ldi ? Heer. 



I'Intc \.\l, Fi-. U. 



Frond unknown; pinna' linear, narrow; pinnules in the lower part of 

 tlie piiuia^ separate to tlio liase, in the upper portion united at base, oblong- 

 obtuse, minute; nerves of tlie i)iiuinles distinct, consisting of a midnerve 

 which sends off alternately lateral nerves, which in the lower part of the 

 pinnules are forked, but in the upi)er portion are simple. 



Locality: Fisliing hut above Dutch Gap Canal; very rare. 



Tliis plant is found only in small fragments. In the absence of fruc- 

 titication its identilication with Gleichenia is not certain. It is very much 

 like G. Nonlnishiijlill I leer.' 



Undetermined fern, sp. nov. 



riato XXI, Fig. 12. 



This small fragment is too incomplete to justify a description. It 

 reminds one of Asplenium Dkksonianum Heer." 



Occurs very rarely at the fishing hut above Dutch Gap Canal. 



THYRSOPTERIS Kunze. 

 Sterile and fertile pinnules dimor})lious; sterile frond decompound; 

 pinnules constricted at base, cut into lobes or teeth ; nerves, lateral ones, 



' Compare Flor. Fobs. Arc, vol. 3, pt. 2, PI. IX, Figs. 6, 12. 

 * Compare Flor. Fobs. Arc, vol. 3, pt. 2, PI. I, Fig. 1. 



