270 SYSTEMATIC PALEONTOLOGY 



tends to give a Mesozoic fades to the Potomac flora. They have almost 

 or quite passed away in the interval betvveen the time of the deposition 

 of the lower Potomac and that of the oldest Cretaceous of ISTew Jersey. 

 Their absence in the flora of the latter group greatly adds to the com- 

 paratively recent aspect of this latter. A number of these Thyrsopterids 

 have the same type of foliage as the Wealden ferns, Sphenopteris Man- 

 telli Brongn. ; S. Goepperti Dunker; S. cordai Schenk; 8. plurinervia 

 Heer; and 8. Go7nesiana Heer, as well as the Urgonian plants Asple- 

 nium Dicksonianum Heer; A. Nauckhoffianum Heer, and various Dick- 

 sonias, such as D. Johnstrupi Heer. It is a significant fact that this 

 type of foliage, so common towards the close of the Jurassic and in the 

 oldest Cretaceous, is the most abundant single type in the Potomac 

 strata also. Such a general prevalence of a type is more significant of 

 geological relationship than the identification of a few species common 

 to two formations. It is not worth while to e^xamine in detail the affin- 

 ities of the different species. Most of them are new and unique. One 

 or two have some resemblance to Oolitic species, while a greater number 

 may be grouped as belonging to the two Wealden types 8. ManieUi and 8. 

 Goepperti/' 



It will be seen from these lengthy quotations how uncertain the author 

 of these forty species of TJiyrsopteris was of their real botanical affinity, 

 and when the student turns from the text and figures to the actual speci- 

 mens, the strictures of Professor Seward (Wealden FL, pt. i, 1894, p. 

 56 et seq.) are abundantly justified. There are twenty-six species de- 

 scribed from a single clay lens at Fredericksburg, Virginia. If the 

 reader will pause to ask himself where in the history of the earth or in 

 the living flora twenty-six species of a single genus of ferns can be found 

 in a single circumscribed clay lens or growing in a single circumscribed 

 area, grave doubt as to their validity at once arises, and even if we predi- 

 cate their having been gathered together by a river system it must 

 needs have been a remarkable river system to have gathered together all 

 of these ferns along mth over fifty other species of ferns and fifty species 

 of gymnosperms, in all one hundred and sixty different species, and de- 

 posited them in one quiet pool where clay was forming, a pool not over 



