NO. 1862. FOSSIL FERNS FROM THE POTOMAC GROUP— BERRY. 323 



are somewhat modified, eo as to depart more or less from the typical S. mantelli, and 

 to assume the facies of Thyrsopteris. The other species of Sphenopteris give little help 

 in fixing the age of the Potomac strata (p. 338). 



Thus while the most prominent fern element in the Potomac group 

 belongs to a different genus and different famUy, its resemblance to 

 the Sphenopteris mantelli type is so pronounced that it furnishes an 

 argument for the nearly homotaxial age of the containing deposits, 

 surely a curious logic. In his latest work this author identifies a 

 species of OnycJiiopsis from three localities in Virginia and Marjdand 

 (Hell Hole, New Reservoir, and Fort Foote). 



Again in discussing Thyrsopteris at the end of his Potomac flora 

 (1890) he writes: 



It is true that, as no fructification has been found on these ferns, they may be incor- 

 rectly placed in the genus Thyrsopteris. Still, the very great development in the 

 Potomac flora of ferns with a foliage and nervation so characteristic of the later Jurassic 

 and Lower Ci'etaceous can not be without significance. 



A number of these Thy rsopterids have the same type of foliage as the Wealden ferns, 

 Sphenopteris mantelli Brongn.; S. goepperti Dunker; 5'. cordai Schenk; S. plurinervia 

 Heer; and S. gomesiana Heer, as well as the Urgonian plants Asplenium dicksonianum 

 Heer; A. nauck hoffianum Heer, and various Dicksonias, such as D. johnstrupi Heer. 

 It is a significant fact that this type of foliage, so common toward 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 rela- 

 tionship than the identification of a few species common to two formations. It is 

 not worth while to examine in detail the affinities 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 S. 

 mantelli and S. goepperti. 



It will be seen from these length}' quotations how uncertain the 

 author of these 40 species of Thyrsopteris was as to their real botanical 

 affinity, and when the student turns from the text and figures to the 

 actual specimens, the strictures of Professor Seward ^ are found to be 

 abundantly justified. There are 26 species described from a single 

 clay lens at Fredericksburg, Virginia. If the reader wdl pause to ask 

 himself where in the history of the earth or in the living flora 26 species 

 of a single genus of ferns can be found in a single circumscribed clay 

 lens, or growing in a single cu'cumscribed area, grave doubt as to their 

 validity at once arises; and even if we predicate their having been 

 gathered together by a river S3"stem it must needs have been a re- 

 markable river system to have gathered all of these ferns with over 

 50 other species of ferns and 50 species of gymnosperms, in all 160 

 different species, and to have deposited them in one quiet pool where 

 clay was forming, a pool not over 15 feet in diameter as preserved and 

 only 4 feet tliick, the recognizable remains practically all coming from 

 the basal 3 to 5 inches. 



1 Wealden Flora, pt. 2, 1894, p. 56. 



