INTRODUCTION. 



XXV11 



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 ; J 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. 2 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 3 : 



Equisetum Lyelli, Mant. = Equisetites 



Lyellij Mant. 

 E. virginicum, Font. 

 Cladophlebis constricta, Font. 

 C.falcata, Font. 

 C. acuta, Font. 

 C. oblongifolia, Font. 

 C. brevipennis, Font. 

 Pecopteris strictinervis, Font. 

 P. constricta, Font. 

 P. Browniana, Dunk. 

 Sphenopteris Mantelli, Brong. = Ony- 



chiopsis Mantelli (Brong.). 



Aftpidium Dunkeri, Schimp. = Clado- 

 phlebis Dunkeri (Schimp.). 



Asplenium dubium, Font. 



Thyrsopteris rarinervis, Font. = Ony- 

 chiopsis elongata (Geyl.). 



T. densifoHa, Font. 



Dioonites Buchianus, Schimp. 



Dioonites Buchianits, var. angusti- 

 folius, Font. 



Tysonia Marylandica, Font. 



Sphenolepidium virgmicum. Font. 



S. Kurrianum, Heer. 



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 et seq. 



