as to the Age of the Potomac Formation. 123 



in four of which the resemblance is close enough to warrant 

 the preservation of the generic name. 



A somewhat larger number of species are compared with 

 those of Franconia and Seinstedt in Germany, though here 

 the affinities are less close. Six species are compared with 

 forms from the well known Khetic beds of Sweden and three 

 with the Jurassic of China where Dr. Newberry found plants 

 identical with those collected by Emmons in North Carolina. 



Taking all these Rhetic localities together we find that four- 

 teen Potomac species have important characters in common 

 with Rhetic plants, although none of them are specifically 

 identical. 



Lias.- — The Rajmahal group of India has generally been re- 

 ferred to the Lias and is the only flora of that age which con- 

 tains types that reappear in the Potomac beds. Six species 

 are compared in three of which the same genera occur, but 

 two of the remainder either present only an accidental resem- 

 blance or there has been a mistake in their generic reference. 



Oolite. — Six Oolite beds contain analogous forms to those of 

 the Potomac These are : 1. The Upper G-ondwanas of India; 

 2. That of Cape Boheman, Spitzbergen ; 3. Those of eastern 

 Siberia ; 4. Those in Russia, worked up by Schmalhausen ; 

 5. Those of Italy monographed by Zigno ; and 6, the cele- 

 brated Yorkshire Oolites. The Siberian and Yorkshire beds, 

 however, have yielded the greater part of the species allied to 

 Potomac plants. Thirty-one species of Potomac plants have 

 allies in the Oolite flora, but as yet there occur none that are 

 identical. 



Upper Jura. — The upper Jurassic deposit at Solenhofen in 

 Bavaria, the Coral beds of Yerdun and St. Mihiel in France, 

 and the Kimmeridge strata at Morestel and the Lac d'Armaille, 

 have each yielded forms, especially of the coniferous genus 

 Brachyphyllum, which closely resemble Potomac specimens. 



Wealden. — The Wealden of England furnishes two species 

 which also occur in the Potomac formation. These are Equi- 

 setum Lyelli JVIant., and Sphenopteris Mantelli Brongn., the 

 latter of which also occurs in the Wealden beds of Germany, 

 first monographed by Dunker. Besides these there occur on 

 the Island of Portland and the Isle of Wight certain Cycadean 

 trunks referred to Cycadeoidea, Bennettites and MantelHa, 

 which are very similar to those found by Tyson in Maryland, 

 called by Professor Fontaine Tysonia Marylandica. 



A much larger number of identical species are found in the 

 Wealden of Germany. Besides the one already mentioned as 

 common to strata of that age in England and Germany, we 

 have Peeopteris Browniana Dunk., P. Dunkeri Schimp., 

 Dioonites Puchianus Schimp., Dioonites abietinns Miquel, 



