3 10 THE POTOMAC OR YODNGEK MESOZOIO FLORA. 



Potomac by a comparison of its plantis with known floras. Its extinction 

 indicates a considerable antiquit}' for the Potomac strata. 



Thyrsopterl'i : What was said of CladopMchis may in the main be 

 repeated for tliis genus. It is true that, as no fructification lias been found 

 on tliese ferns, they may be incorx-ectly 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 Cre- 

 taceous can not be without significance. This type of fern is much the 

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

 acterized, number forty. They are distributed over the whole of the Poto- 

 mac area, and a number of them are among the most common ferns at the 

 localities yielding them. This group, more than any other, tends to give a 

 Mesozoic facies to the Potomac flora. They have almost or quite passed 

 away in the interval between the time of the deposition of the lower Poto- 

 mac and that of the oldest Cretaceous of New Jersey. Their absence in 

 the flora of the latter group greatly adds to the comparatively recent 

 aspect of this latter. 



A number of tliese Thyrsopterids have the same type of foliage as the 

 Wesilden krna, S2)heno2}teris ManteUi Brongn.; S. Ga^^jjerii Dunker; S. cor- 

 dai Sclienk ; S. plurinervia Heer ; and *S'. Gomesiana Heer, as well as the 

 Urgoniau plants Asplen'mm Dkksonianum Heer; A. Nauckhoffianmn Heer, 

 and various Dickson las, such as D. Johnstnqn 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 examine in detail the affinities of 

 the diff'erent 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. ManteUi and S 

 Goepperti. 



Osmunda: The plants placed in the genus Osmunda, although not 

 numerous in species, are very common at the Potomac Run locality. 

 They quite possibly ditfer somewhat from true Osmundas, the genus not 



