POTOMAC FOEMATION IN VIRGINIA AND MARYLAND. 591 



the meager data furnished by the specimens, the identity of the species 

 is questioned, this is also indicated by an interrogation point opposite the 

 number in the appropriate cokimn and Une of the table. As most of 

 these are probably correct, I have thought best to treat them so in the 

 analysis of the table, since the few cases in which they may be incorrect 

 could not seriously affect the general results. 



It thus appears that the 176 species are represented by 2,882 speci- 

 mens, or a mean of a little over 16 specimens to each species. This of 

 course is mainly due to a few species that are abundantly represented, 

 though 47 species occur in only a single specimen. A still larger number 

 have only two or three specimens. Still, 38 species are represented by 

 16 specimens or more, and a large number range from 10 to 25 specimens. 

 The most abundant species are : 



Cladophlebis acuta, with 400 specimens. 



Athrotaxopsis expansa, with 234 specimens. 



Sphenolepidium Sternbergianum densifohum, with 186 specimens. 



Thyrsopteris rarinervis, with 152 specimens. 



Vitiphyllum multifidum, with 151 specimens. 



Sapindopsis variabilis, with 132 specimens. 



Cladophlebis acuta angustifolia, with 115 specimens. 



All the rest have fewer than 100 specimens, but Dryopteris parvifolia 

 has 71, Glyptostrobus (Taxodium) brookensis 65, and Acrostichopteris 

 longipemiis 57, while eleven others have between 30 and 50 specimens. 



The collections were made at 42 localities, but the number of both 

 species and specimens from the different localities differs even more widely 

 than does the number of specimens of the different species. The localities 

 yielding the largest number of specimens are: Arlington, with 748; 

 Federal Hill, with 324; Langdon, with 303; Hosiers Bluff, with 279; the 

 Mount Vernon beds, with 230; Chinkapin Hollow, with 210; Vinegar 

 Hill, with 129; Muirkirk, with 99; the new reservoir, with 97; White 

 House Bluff, with 64; Hell Hole, with 57; and Cockpit Point, with 55. 

 The rest all yielded fewer than 50 specimens each, and 8 localities are 

 represented by a single specimen each. 



Of the localities in Virginia besides those representing the Mount 

 Vernon chocolate clays, the following have been discovered since the 

 appearance of Professor Fontaine's monograph: Alum Rock, the bed at 

 the north end of the 72d Milepost cut. Cockpit Point, Woodbridge, the 



