OLDER POTOMAC OF VIRGINIA AND MARYLAND. 539 



In the different collections there are fully 400 determinable specimens. 

 They are all fragments of pinnse, mostly ultimate ones, with pinnules in 

 various states of preservation and from different portions of the frond. 

 In this large amount of material it is to be expected that more variation 

 would be found than was shown in the Virginia specimens. Still, the 

 species appears remarkably constant. A very small number of the 

 specimens show on some of the largest pinnules a slight crenulate toothing. 

 Some of the pinnules are larger than any in the Virginia forms. Some of 

 them are narrower than the normal ones and approach in that respect 

 the variety called in this paper angustifolia. Some of the larger and longer 

 pinnules resemble C. falcata. This latter, however, quite constantly, 

 has the lateral nerves of the pinnules deeply bifurcate. In C. acuta they 

 are simply furcate. Only in a very few of the largest pinnules are they 

 sometimes bifurcate. The lateral nerves in C. acuta are quite constantly 

 forked near their insertion on the midnerve. They then diverge strongly 

 and become subparallel before reaching the margin of the pinnule. PI. 

 CXIV, Fig. 3, gives some of the larger pinnules, and Fig. 4 some of 

 unusual length in proportion to their width. The former of these is No. 

 5375 and the latter No. 5120 of the Woman's College of Baltimore. 



Cladophlebis acuta angustifolia Fontaine n. var. 

 PL CXIV, Fig. 5. 



One of the most common plants at the Arlington localities is a fern 

 with narrow pinnules, which in most respects closely resembles C. acuta. 

 It differs from the latter only in the form of its pinnules. These are 

 decidedly narrower in proportion to their length than those of the normal 

 form. It is true that in some specimens the pinnules of the normal 

 form, in some portions of the pinnte, approach these in narrowness, but 

 there are too many of these narrow ones and they are too constant in 

 character to be regarded as sporadic variations in the normal type. 

 These forms I'esemble also Dryopteris angustipinnata, presently to be 

 treated. From this also it differs by constant features, which will there 

 be pointed out. 



In the collections made from the Arlington localities there are 115 

 specimens of this form. The variety angustifolia differs from the normal 

 form in having pinnules narrower in proportion to their length and in 



