GEOLOGICAL AFFINITIES OF THE I'OTO.MAC PLANTS. iiSd 



witli the older pre-Cretaceous floras. But their presence is not certainly 

 established, and, if" present, their .species are too few and feel)!}' devchipetl 

 to indicate anything more tlian a recent introduction. 



Acrostichopteris seems to be an extinct genus, peculiar to the Potomac 

 formation. It can then give but little aid in fixing geological age. The 

 fact, however, that it is extinct prolial)!}' denotes a comi)aratively great age 

 for the formation to which it is confined. 



Aspleniiun: This genus also, if present, is probaltly to be considered 

 as a recent element, but newly introduced. 



Tliinnfeldki: This genus shows three species; one of them, TlibnifrJdia 

 variahiUs, being quite common at the 72d mile-post, near Brooke. The genus 

 is an old one, characterizing the Rluetic, and Infra-Lias. The continuance 

 of this type into the Potomac formation makes an important addition to the 

 older elements of the flora. The species, although not capable of identifi- 

 cation with any of those of the older floras, nuist be regarded as survivors. 



Stenopteris : This genus is doubtfully present. It is too rare and too 

 imperfectly preserved to be of use if present. It is one of the still linger- 

 ing Jurassic types, now almost extinct. 



Angiopteridiwn : This is a more valuable genus in throwing liglit on 

 the age of the flora containing it. The absence of fructification in the 

 Potomac plants, however, renders it impossible to say positivel}' whether 

 or not these forms are really Angiopteridimn ; but it is at least clear that 

 large ferns with this type of foliage are more characteristic of the older 

 Mesozoic formations than of the later ones. While the number of indi- 

 viduals of any pai'ticular species is very small, yet the number of species 

 is surprisingly large, and the genus adds much to the archaic facies of the 

 flora. This is one of the groups which does not appear to go uj) into the 

 Cenomanian flora, and hence, like Cladophlehis, it must be regarded as a 

 still surviving older element. 



Gleichcma: This genus, which, as Ileer has shown, is so largely rep- 

 resented in the older Cretaceous of the northern part of the American con- 

 tinent, shows a surprising deficiency in forms in the Potomac flora. 



Aspleniopfcris : This is an extinct form peculiar to the Potomac beds, 

 and for that reason it can give us little help in determining the age of the 



