346 THE POTOMAC OR YOUNGER MBSOZOIC FLORA. 



species, although new, have most affinity with species from formations of 

 those ages. 



Sphenolcpidium : This is one of the most important coniferous genera 

 in the Potomac. It gives two characteristic genera of the Wealden abun- 

 dant in the Potomac, viz, -S". Kurrianum and 8. Sternhergianum. The large 

 development of this type adds much to the weight of evidence for the 

 Neocomian age of the Potomac. 



Ahieiites: The type of cones placed in AhietUes is too poorly preserved 

 and too rare to have much value; but, so far as their evidence goes, it 

 indicates an age more recent than the lower Cretaceous. 



We may j^ass over the various forms of fruits and inflorescence, and of 

 course the undetermined plants, as too poorly characterized to throw light 

 on the question of age, and take up the angiosperms. 



ANGIOSPERMS. 



The angiosperms, in the number of kinds and in the degree of devolop- 

 ment of some of the species, are pretty well represented. Hitherto it has 

 proved so uniformly true that, with the single exception of Heer's Populiis 

 2irimcBva from Kome, angiosperms are totally absent from all Neocomian 

 floras, that Saporta is inclined to think that this Populus was erroneousl}' 

 ascribed to Kome. Any considerable number of angiosperms in a flora 

 would, perhaps by most if not all paleobotanists, be taken by itself as almost, 

 if not quite, proving the j^ost-Neocomian age of the flora which contains them. 



Now, for the first time, we have presented in a flora which possesses 

 an overwhelmingly large proportion of Jurassic or typical Mesozoic 

 elements a very considerable number of angiosperms. The presence of 

 these plants, taken by itself, should not induce any one to assume that the 

 Potomac flora is post-Neocomian. It simply confirms the assumption 

 made by more than one writer, that the apparently sudden advent of this 

 type of jilants in pi-edominating numbers in the Cenomanian is not to he 

 taken as indicating their true mode of appearance. Perhaps all paleobot- 

 anists will admit that angiospenus really made their appearance gradually, 

 and that their first advent must be looked for far back in the Mesozoic, in 

 the midst of a floi'a essentially Jurassic or Mesozoic in type. 



