168 BENEDICT: GENERA OF THE FERN TRIBE VITTARIEAE 
always more than two rows of areolae. Of this pluriseriate type 
two distinct patterns occur in the tribe, the distinction depending 
upon the presence or absence of a midrib (PLATE 6, FIG. 1 and 2). 
That this distinction is important is evidenced by the distri- 
bution of the species. All the American species possess a distinct 
primary vascular strand, a complete midrib, in relation to which 
the areolation is secondary and derivative. In the Old World 
species, on the contrary, there is no complete primary axial vein. 
There occurs in some few species of this region an incomplete 
midrib, but in none of these species is this midrib ever maintained 
as predominant throughout the leaf, and in all the Old World 
species the areolation is in part or entirely derived by the dichot- 
omy of the several veins present at the lower part of the blade. 
As further evidence of the fundamental character of this 
distinction in venation, shown in the mature sporophytes, the 
fact should be noted that among the American pluriseriate species 
three distinct genera, based on sporangial arrangement, are 
recognized; that is to say, subsequent to the separation of the 
continental groups on the basis of venation, one of these groups 
has been further differentiated into three well-marked genera. 
Surely continental venation differences in this case deserve pre- 
cedence over intracontinental soral variations. Additional evi- 
dence is offered farther along in connection with the description of 
the ontogenetic stages. 
Notwithstanding, however, this accepted generic separation 
within one venation group, the larger number of American pluri- 
seriate species have been retained under the same generic name as 
the Old World species, viz., Antrophyum Kaulf. The principal 
reason for such retention under one name has been the fact that 
the soriation is somewhat similarly indefinite in both groups. But 
it should be apparent, I think, as expressed above, that as a generic 
character the venation pattern must take precedence over the spor- 
angial arrangement, and that Anirophyum as at present delimited, 
includes two generic groups more distinct from each other than 
are the three pluriseriate groups which are now kept apart because 
of different sporangial arrangement. 
For this reason the American species formerly classified under 
the name Antrophyum are here included under a different name, 
