DESCRirXlON OF TUE aPEClES. 83 



to take out the speciuiens without reducing their size very considerably. 

 In PI. VIII, Fig- 2, is shown a fragment of what was formerly a portion 

 of the frond a foot wide and more than a foot long. At all tlie other 

 localities it is rare. It is a singular fact that not a trace of this plant is 

 found at the locality on the hill-side, which is not more than one hundred 

 and fifty }ards distant from that on the road-side. This is true, although 

 ferns are the most common fossils at the first-named place. 



This fern is so well characterized, that there is no difficulty in identify- 

 ing it. Indeed it can usually be detected at a glance. The plant shows 

 some variation. Some of the pinnules, especially in those specimens found 

 at the fishing hut above Dutch Gap Canal and at Baltimore, are very 

 narrow, elongate, and slightly toothed. We may take PI. VIII, Fig. 6, as 

 a typical specimen, the teeth being acute; but in Fig. 1 of the same plate 

 we have a plant from Fredericksburg wliich has the teeth less deeply cut 

 and more obtuse. This form somewhat resembles Cladophlehis ohioiifja. 



In most cases, and in typical forms, the pinnules are narrowed at 

 base, strongly decurrent, and united to form a wing, as in PI. VIII, Fig. 4, 

 from Fredericksburg, and Figs. 2, G ,7 of the same plate, coming from the 

 road-side near Potomac Run. In such forms, however, as PL VIII, Fig. 3; 

 PI. IX, Fig. 1, from Fredericksburg, and Fig. 5 of the same plate, from 

 the 72d mile-post, as well as PI. IX, Fig. 2, from the fishing hut above 

 Dutch Gap Canal, the pinnules are separate, not decurrent at base, less 

 linear, and narrow in form, and less inclined forward. These points may 

 be due to the fact that the parts which show them come from a portion of 

 the frond different from that affording the normal forms, or tliey may be 

 of sufficient importance to denote a variety of the species. 



The form, PI. VIII, Fig. 5, from the locality at the 72d mile-post, is 

 the only specimen which gives the summit of a compound pinna. The 

 corded mai-gins of the rachis appear only on the upper surface, as in 

 PL VIII, Fig. 2, while the lower face is rounded and prominent, as in 

 Figs. 1, 4, G of the same plate. The lateral nerves are typically once 

 forked only, as in PL VIII, Figs. 2», 2\ but in PL VIII, Fig. 3, and PL 

 IX, Fig. 1, they in the lower teeth form nerve-groups, composed of lateral 

 nerves which "-o oft" alternatelv from a midnerve. 



