as to the Age of the Potomac Formation. 



127 



viz : Equisetum LyeUi Mant., Pecopteris Browniana Dunk., 

 and Dioonites abietinus Miquel, which Prof. Fontaine treats 

 as a variety of Dioonites Buchianus Schimp. Only one spe- 

 cies is confined to the Urgonian, viz : Sequoia gracilis Heer, 

 and only two to the Cenomanian, viz : Pecopteris socialis 

 Heer and Aspidium Oerstedi Heer, both from the Atane 

 beds of Greenland. The remaining ten identical species range 

 through more than one series, most of them through several. 

 Two species are found in the Wealden and Neocomian, viz : 

 Sphenopteris Mantelli Brongn., and Pecopteris Dunkeri 

 Schimp, which Prof. Fontaine regards as a true Aspidium. 

 One Wealden species, Dioonites Buchianus Schimp, is found 

 in both the Urgonian and the Cenomanian, and two, Spheno- 

 lepidium Kurriamim Heer, and S. Sternbergianum (Dunk) 

 Heer, are found in the Wealden, Neocomian and Senonian. 

 One species, Gleichenia Nordenskioldi Heer, occurs only in 

 the Urgonian and the Dakota Group ; another, Sequoia am- 

 bigua Heer, only in the Urgonian and Cenomanian of Green- 

 land. Sequoia rigida Heer, occurs in the Urgonian, Gault, 

 Cenomanian and Senonian, while Sequoia Peichenbachi (Gein.) 

 Heer, runs through the Urgonian, Gault, Cenomanian, Dakota 

 Group, Senonian, and is even found in the Laramie Group. 

 The only species not already mentioned is Sequoia subulata 

 Heer, which is found only in the Cenomanian and Senonian 

 of Greenland. 



These results can be shown in tabular form, and, if the spe- 

 cies be arranged in ascending geological order they will stand 

 as follows : 



Species common to the Potomac and 

 other Formations. 



Equisetum Lyelli Mant 



Pecopteris Browniana Dunk. 



Dioonites abietinus Miqnel. 



Sphenopteris Mantelli Brongn. _ 



Pecopteris Dunkeri Schimp 



Sphenolepidium Kurriamim Heer 



Sphenolepidium Sternbergianum (Dunk.) Hr 



Dioonites Buchianus Schimp 



Sequoia gracilis Heer. 



Sequoia ambigua Heer 



Sequoia rigida Heer. 



Sequoia Beichenbachi (Gein.) Heer. 



Gleichenia Nordenskioldi Heer. 



Pecopteris socialis Heer. _ 



Aspidium Oerstedi Heer. _ 



Sequoia subulata Heer. 













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