Maryland Geological Survey 269 



the species of Thyrsopievis described in the following pages show a good 

 many features similar to those of Sphenopteris Mantelli, as described by 

 Schenk and Heer" (p. 120). 



Professor Fontaine does identify Sphenopteris Mantelli from one lo- 

 cality in the Potomac belt, that at Federal Hill, Baltimore, and in dis- 

 cussing its bearing upon the age of the deposits he says: 



" Now in the Potomac flora not only is S. Mantelli present in beds 

 which show plants of the most recent facies existing in the formation, 

 but there is a very important group of ferns which, although placed in 

 the genus Thyrsopteris^ have nearly the nervation and foliage typified in 

 S. Mantelli. The great development in the Potomac of ferns of the 

 general type of S. Mantelli gives strong evidence of Weal den or some- 

 what later age. A somewhat later age than Wealden is indicated, per- 

 haps, as most of the species are somewhat modified, so as to depart more 

 or less from the typical S. Mantelli, and to assume the facies of Thyrsop- 

 ievis. 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 family, its resemblance to the 

 Sphenopteris Mantelli type is so pronounced that it furnishes an argu- 

 ment for the nearly homotaxial age of the containing deposits, surely a 

 curious logic. In his latest work this author identifies a species of 

 Onychiopsis from three localities in Virginia and Maryland (Hell Hole, 

 3 sp.; New Eeservoir, 1 sp., and Fort Foote, 2 sp.). 



Again, in discussing Thyrsopteris at the end of his Potomac flora 

 (1890), he writes: "It is time that, as no fructification has been found 

 on these ferns, they may be incorrectly 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 

 Cretaceous cannot be without significance. This type of fern is much 

 the most common in the Potomac strata. The species, most of them 

 well characterized, number forty. They are distributed over the whole 

 of the Potomac area, and a number of them are among the most common 

 ferns at the localities yielding them. This group, more than any other, 



