268 Systematic Paleontology 



pinnse alternate, approximate, lanceolate. Pinnules narrow, lanceolate, 

 acute, alternate, the larger ones serrate, and gradually passing into pin- 

 nules with narrow ultimate segments. Fertile pinnse with alternate 

 elliptical pinnules which differ in shape from the sterile pinnules and 

 have the sporangia on the lower surface, giving them the appearance 

 of raised elliptical bodies." 



The most abundant and characteristic ferns of the Potomac G-roup 

 were referred by Professor Fontaine to Thyrsopteris Kuntze, an existing 

 monotypic genus of the family Cyatheacese inhabiting the island of Juan 

 Fernandez. Of these some forty species, so called, were described. They 

 were all based on sterile fronds or parts of fronds, often extremely small 

 and inadequate fragments. Professor Fontaine, after quoting Heer's 

 diagnosis of Thyrsopteris^ writes : 



" This description, given by Heer for the genus Thyrsopteris, so far 

 as the portion pertaining to the sterile frond is concerned, agrees well 

 with a large number of species in the Potomac flora. These I place 

 provisionally in the genus Thyrsopteris, on account of the great re- 

 semblance that the shape of the pinnules, the lobing, and the nervation 

 show to the sterile forms of various species determined to be Thyrsopteris 

 by their fructification. As, however, no fructification is found in the 

 Potomac species, the placing of these plants in the genus must be re- 

 garded as provisional. It is quite possible that some of them belong to 

 Aspidium and Dicksonia. The genus Thyrsopteris seems to be eminently 

 a Jurassic type of fern. However, it may prove to be with the fructifi- 

 cation of the various species assigned in this memoir to this genus, it is 

 probable that they are at least distinct and true species. The great va- 

 riety and comparative abundance of these forms show that this type was 

 developed in the Potomac flora to an unusual degree. They surpass all 

 other types, even that of Cladophlehis, and give a decided Jurassic facies 

 to the assemblage of ferns found in this flora. Even if we cannot by fruc- 

 tification determine the affinities of fossils with living plants, the large 

 development of sterile forms of well-marked types is of importance in 

 fixing the character of a flora. It should be noted that a number of 



^ Heer, Fl. Foss. Arct., Band iv, AMh. ii, 1877, p. 28. 



