194 THE rOTOMAC OR YOUNGER MESOZOIC FLORA. 



generally diffused than are most of the species of ferns. When a species 

 of fern is found it is always, except on Potomac Run, among the rarest of 

 the plants. At the same time the different species of ferns are usually 

 very restricted in occurrence. The conifers form the most widely diffused 

 of the Potomac plants, and when they occur they are generally the most 

 abundant plants at the locality. One or two examples may be given to 

 illustrate the abundance of individuals. At the entrance of Trent's Reach, 

 in one layer six to eight inches thick, fragments of the branchlets of 

 Fenelopsis parceramosa. were so abundant, that they formed a considerable 

 portion of the mass of the stratum. With these immense numbers of 

 fragments of Baleropsis pleiiripartUa were found. Fenelopsis ramoslssUna is 

 by far the most common plant at Fredericksburg. 



The abundance of imprints of the leaves of conifers is not the only 

 evidence of the great development of conifers in the Potomac. It is shown 

 in the immense amount of lignite formed from coniferous wood. This 

 lignite occurs both in the form of isolated logs and fragments, and in tiers 

 of logs piled one over the other, as if caused by prostrate forests, indi- 

 cating that some, at least, of the conifers of that day grew massed in forests 

 as now. 



Certain peculiar genera, now extinct, seem to have been very largely 

 developed in the Potomac vegetation, and if we are to judge from their 

 predominance in the fossils of localities where they occur, they nuist have 

 formed the leading and most characteristic t3'pes. These will be noticed 

 further on under proper heads. The archaic character of many conifers 

 as well as ferns does not prepare us to expect to find angiosperms asso- 

 ciated with these two elements. 



NAGEIOPSIS, gen. nov. 



Trees or shrubs with leaves and branches spreading in one plane ; 

 leaves varying much in size and shape, those towards the base of the twigs 

 sometimes smaller than those higher up, distichous mostly, or rarely sub- 

 distichous, opposite and persistent, attached by a short slightly twisted 

 footstalk, usually to the side of the twig, more rarely slightly within the 



