POTOMAC FORMA riON IN VIKCilNIA AND MAKVLAND. 591 



the ineafKM' data t'uniislR'cl l)y the spiriineiKS, the identity of the speeies 

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

 number in the appropriate eokunn and Hne of the table. As most of 

 these are probably eorrect, 1 have thoujiht best to t!(>at them so in the 

 analysis of th(> table, since the few cases in which they may l>e incorrect 

 could not seriously aflect the general results. 



It thus appears tliat the 176 species are re])resented l)y 2,882 speci- 

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

 course is niainl>- (lu(> to a few species that are abundantly represented, 

 though 47 species occur in oiil\- a single .specimen. .\ still largei' numl)er 

 have only two or three specimens. Still, 38 species are r(>piesented by 

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

 The most abundant species are: 



Cladui)lil('l)is acuta, with 4UU hipeciinens. 



Athrotaxoj)sis expansa, with 234 speciinen.s. 



Sphenolepidium Sternbergiamim densifoliuni, with is(i specimens. 



Thyrsoptoris rarinervis, with 152 specimens. 



Vitiphylluni niultiflthun, with 151 specimens. 



Sapindopsis variabilis, with 132 specimens. 



Cladophlebis acuta angustifolia, with 115 specimens. 



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

 has 71, Glyptostrohus (Taxodimn) hrookensis 65, and Acrof<fichoptcris 

 iongipennia bl , 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 diffei-s 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 \'ernon Ijeds, with 230; Chinkapin Hollow, with 210: Vinegar 

 Hill, witli 129; Muirkirk, with 99; the new reservoii', with 97: White 

 House Hluff, 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 Mrginia besides those representing the Mount 

 Vernon chocolate clays, the following have been discovered since the 

 appearance of Professor Fontaine's monograph: Alum Rock, the ])ed at 

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



