A REVISION OF THE FOSSIL FERNS FROM THE POTOMAC 

 GROUP WHICH HAVE BEEN REFERRED TO THE GENERA 

 CLADOPHLEBIS AND THYRSOPTERIS. 



By Edward W. Berry, 



Of the Johns Hopkins University , Baltimore. 



The present paper is the fourth and last of a series of revisions of 

 the more important genera of fossil plants from the Potomac group 

 in Maryland and Virginia.^ The following genera have been discussed 

 in previous numbers of the Proceedings : Nageiopsis, AcrosticJiopteris, 

 Taeniopteris, Nilsonia, Sapindopsis, Sequoia, Athrotaxopsis, Sphe- 

 nolepis, Ahietites, Pinus, CepJialotaxopsis, BracJiyphyllum, and Wid- 

 dringtonites. 



With the appearance of the present communication all of the larger 

 genera including those most in need of revision will have been treated. 

 The remainder of the flora, numbering about one hundred species, will 

 be fully described and illustrated in a Monograph of the Lower Creta- 

 ceous which will appear under the auspices of the Maryland Geological 

 Survey. 



The fern genus Cladoplilebis, wliich is discussed in the first part of 

 this paper, is an important cosmopolitan type in the Lower Cretaceous, 

 a type which is an undiminished survival from the older Mesozoic. 

 A large number of species have been described, both in this country 

 and elsewhere. These ferns are abundant and important elements 

 in the Potomac flora. The remains represent for the most part 

 species of considerable size. The usual difficulties in dealing with 

 fossil fragments of large and somewhat variable (in time and space) 

 fronds have resulted in the previous description of many more species 

 than the evidence warranted, a result equally confusing to both the 

 botanist and the geologist. 



The numerous species of Thyrsopteris recorded in the Uterature 

 from the Maryland- Virginia area are treated in the second part of 



iThe previous papers are: (1) A revision of the fossil plants of the genus Nageiopsis of Fontaine, 

 Proc. U. S. Nat. Mus., vol. 38, 1910, pp. 185-195. (2) A revision of the fossil plants of the genera 

 Acrostichopteris, Taeniopteris, Nilsonia, and Sapindopsis from the Potomac group, Proc. U. S. Nat. 

 Mus., vol. 3s, 1910, pp. 625-644. (3) A revision of several genera of Gymnospermous plants from the 

 Potomac group in Maryland and Virginia, Proc. U. S. Nat. Mus., vol. 41, 1911, pp. 289-318. 



Proceedings U. S. National Museum, Vol. 41, No. 1862. 



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