66 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



Klender decorticated branches, often attached to stems but sHghtly lai'ger 

 tlian the brandies, in such a manner as to indicate tliat the branching was 

 copious ; in tliis respect reminding one of E. Vircjhucum. Tlie branches 

 average in diameter l'"" and under. Considering their slenderness, they 

 have a remarkably rigid and woody aspect. The length that many of 

 these impressions attain and their stiff character remind one of Casuarina. 



Rhizome of Equisetum, sp.? 



Plate CLXX, Fig. 8. 



PI. CLXX, Fig. 8, gives a specimen found at Covington street, Balti- 

 more. It seems to be the decorticated rhizome of some Equisetum, show- 

 ing two tubercles. Only one specimen was found. As E. Marylandicum 

 occurs at this spot, it is probable that this rhizome belongs to it. 



FILICES. 



The ferns form a very important element in the Potomac flora. Not- 

 withstanding the great variety of forms, with very few exceptions speci- 

 mens of ferns are rare at all the localities. This comparative scarcity of 

 individual preserved fossils is no doubt due to the conditions attending the 

 fossilization, which favored the preservation of other plants in larger pro- 

 portion. The large number of species, although represented by compara- 

 tively few individuals, shows that ferns abounded. There is a curious 

 mingling of older and more recent aspects in the fern-flora. Taken as a 

 whole, Jurassic and Wealden types decidedly predominate. The very 

 large proportion of species belonging to the Cladophlehis and Thyrsopteris 

 forms gives a marked Jurassic facies to the flora. Many forms are closely 

 allied to Sjjhciiopfcris Mantdli and other Wealden species, so that the Weal- 

 den element is a large one. But the luunerous species of Aspidium give 

 modern features a strong representation. 



In the ferns, as in most of the other plants of the Potomac, we find 

 nuich isolation of the species, forms occurring at a given locality being 

 commonly restricted to it. In some of the species of Cladophlehis we find 

 some of the closest approximations made by Potomac forms to those of the 

 older Mesozoic or Rhtetic flora of the Richmond Coal Felds. This latter 



