122 L. F. Ward — Evidence of the Fossil Plants 



Elements of the Potomac Flora. 



Number of Genera 



" New Genera 



" Species 



!t New Species 



Species founded on leayes, fruits, etc. 



" ' ; internal structure 



" occurring elsewhere (identical) 



" allied to previously known species. 



" admitting of comparison 



" not admitting of comparison 











CO 













a 





C3 



m 



'B 



to 



a 



o 



w 



a 

 o 



m 

 Pi 



o 



1 



o 



1 



.9 



<D 



o 



w 



to 



O 



O 



W 



t> 



x 



19 



11 



19 



29 



1 





3 



2 



7 



19 





3 



139 



28 



112 



76 



12 



9 



133 



26 



105 



76 



12 1 



3 



139 



28 



107 

 5 



76 



12 



1 



6 



2 



1 







1 



45 



8 



26 



17 



1 



2 



51 



10 



33 



19 



1 



1 



88 



18 



79 



59 



11 



SO 



31 



370 



354 



365 



5 



16 



98 



114 



256 



I have prepared the foregoing table, showing the principal 

 elements of the Potomac Flora as elaborated by Professor 

 Fontaine and Professor Knowltou, from which it appears that 

 the whole number of species thus far known is 370, consisting 

 of 3 species of Equisetum, 139 ferns, 28 cycads, 112 conifers, 

 76 dicotyledons, and 12 species whose systematic position is 

 not definitely known. Of this number, 354 are new species, 

 leaving only 16 species which are common to the Potomac and 

 other formations. But in comparing these new species with 

 the other fossil floras of the globe, 98 of them have been found 

 to resemble to a greater or less extent species figured from 

 elsewhere, leaving 256 species, or nearly 69 per Cent, which do 

 not sufficiently resemble known forms to suggest any close rela- 

 tionship. It is therefore only the 16 identical and 98 presum- 

 ably allied species, or considerably less than one- third of the 

 whole flora, that we have to do with in our comparison of spe- 

 cies. The forms with which these 114 species are found to 

 possess characters in common are derived from 37 distinct flo- 

 ras, ranging from the summit of the Trias to the base of the 

 Tertiary, but chiefly within Jurassic and Cretaceous strata. 



Bheiic. — The formation called Rhetic, which some regard as 

 the base of the Jura and others as uppermost Trias, is the 

 oldest that has yielded forms which are comparable with any 

 found in the Potomac formation. Such forms are found at 

 three, and if we include certain beds in China, at four different 

 localities widely separated from one another, which have been 

 correlated as of Rhetic age. The most interesting of these is 

 that of the Richmond Coal Field in Virginia which Professor 

 Fontaine has himself elaborated, and in which he believes he 

 finds the ancestral types of some half dozen Potomac species, 



