GENERAL REMARKS AND CONCLUSIONS. 331 



Sticrcvlia L. [ Hyjies.ea L. 



Pn(:r. Pa;:o. 



348. S. olegaus, sp. nov ;!lt ' 357. II. Virginicnsis, sp. nov 3".i0 



-VitALiA L. ACKitiriiYl.LrM, gen. nov. 



3.')^. A. iiralioidcs, .sp. nov 321 



ME.NISI-EltMlTK.S Le.S(|. 



359. M. Virgiiiionsis, sp. nov 321 



349. A. tlnbia, sp. nov 314 



JC'CLANDirUVLLUM, gCU. UOV. 



350. J. integrifoliuiii, .sp. nov 315 3£0. M. teuuiiiervis, sp. uov 322 



MvRic.KriiYLLUM, gen. nov. 



351. M. ilentatnin, sp. nov 310 



Platanopiiyllum, gen. nov. 



352. P. crassinerve, sp. uov 316 



ARlSTOLOCni.EPIIYLLrM, gCB. nov. 



301. A. crassinerve, sp. nov 322 



HEDER.EPHYLLU.>r, gen. nov. 



362. H. crennlatuin, sp. nov 324 



363. II. augulatuni, sp. uov 324 



Arali.ephyllum, geu.nov. i Eucalyptophyllum, gen. nov. 



353. A. obtusilobum, sp. uov 317 304. E. oblongifolium, sp. nov 325 



354. A. acatilobum, sp. nov 318 1 _ _ 



„,, . .... „,- I Phyllites, Brongu. 



355. A. luagnifolium, sp. uov 318 ° 



356. A. aceroides, .sp. uov 319 365. P. pacbypliylhis, sp. nov 325 



Of these plants a considerable number may be omitted in any compari- 

 son witli other tlora.s, since they do not present any features that can aid 

 in identification. When, however, these have been omitted, there still re- 

 main many that are well characterized, and that are largely represented 

 and widel}' diffused. These may be taken as characteristic of the flora, 

 and they should have more weight than any others in determining- the age 

 of the strata whicli contain them. 



Before the examination of the geological relations of the Potomac 

 flora is undertaken it will be well to define in what sense the names of cer- 

 tain geological formations, to which fretjuent reference must be made, will 

 be taken in this memoir. The two formations which are capable of mis- 

 conception are the Wcalden and Xcocoiii'koi. By some the Wealden forma- 

 tion is regarded as an independent group forming the uppermost member 

 of the Jurassic. Others regard it as a series of beds contemporaneous with 

 a portion of the lower Xeocomian, formed in estuaries and marshes at the 

 time when a ])ortion of the typical lower Neocomian, which is marine, was 

 being deposited in the sea. The latter view is the one assumed in this 

 memoir. 



In this n-ork tlie Neocomian period is taken as including the Urgonian 

 and Aptian of lJ'Or])igny, the former being, when present, the middle 



