INTRODUCTION. XXVIl 
was first used by McGee on account of the development of typical 
rocks of this series along the course of the Potomac river. Most of 
the plants from these strata were collected from Virginia, a few 
also from Maryland. Fontaine considers it very probable that the 
Potomac beds were laid down under conditions very similar to 
those characteristic of the European Wealden sediments ;’ he 
shows, too, that there is an important floral element in the 
Potomac vegetation which finds its nearest representative in the 
Wealden rocks of Europe. The same author expresses astonish- 
ment at the unusually large number of new species which his 
researches have brought to light in the Potomac flora.* Many of 
the specimens which he has figured are without doubt new 
forms; but, in looking carefully over the numerous illustrations, 
it is difficult, in some cases, to appreciate the minute differences 
which are pointed to as evidence of specific distinctions, and the 
impression is left that the list of new species is perhaps rather 
longer than it need have been. Be this as it may, the 
flora is of exceptional interest, and throws fresh light on the 
difficult problem of the evolution of Dicotyledons. 
The following species are quoted by Fontaine as identical with, 
or closely allied to, European Wealden plants * :— 
Equisetum Tyelli, Mant. = Equisetites | Aspidiwn Dunkeri, Schimp. = Clado- 
Lyelli, Mant. phlebis Dunkeri (Schimp.). 
E. virginicum, Font. Asplenium dubium, Font. 
Cladophlebis constricta, Font. Thyrsopteris rarinervis, Font.= Ony- 
C. falcata, Font. chiopsis elongata (Geyl.). 
C. acuta, Font. T. densifolia, Font. 
C. oblongifolia, Font. Dioonites Buchianus, Schimp. 
C. brevipennis, Font. Dioonites Buchianus, yar. angusti- 
Pecopteris strictinervis, Font. folius, Font. 
P. constricta, Font. Tysonia Marylandica, Font. 
P. Browniana, Dunk. Sphenolepidium virginicum, Font. 
Sphenopteris Mantelli, Brong.=Ony- | S. Kurrianum, Heer. 
chiopsis Mantelli (Brong.). S. Sternbergianum (Dunk.), Heer. 
The flora, as a whole, Fontaine considers to range from the 
1 Potomac Flora, p. 62. 
2 Ibid. p. 334. 
3 pp. 350 ef seg. 
