Silvery Spleenwort 105 
A. thelypteroides, A. filix-femina, and A. cyclosorum are 
undoubtedly congeneric. In habit and general character they 
are more nearly related to the group of Dryopteris of which D. 
noveboracensis, D. thelypteris, and D. simulata are represent- 
atives than to some species of Asplenium. A. thelypteroides is so 
named from its resemblance to D. thelypteris, and D. simulata 
from its simulation of a form of A. jilix-femina. Shorten 
athyrioid sori sufficiently and we have dryopterioid sori: some 
of the short athyrioid sori often seen, on leaves of A. cyclosorum 
and A. filix-femina particularly, are not distinguishable from 
sori of Dryopteris. 
The evolution of Dryopteris from an ancestor with asplenioid 
sori, and also the manner in which the change in the sori may 
have come about, is thus suggested. For instance, let us suppose 
that athyrioid sori were produced first in some Asflenium by 
casual union of otherwise normal asplenioid sori, as they are 
apparently produced in A. thelypteroides, A. filix-femina, and A. 
cyclosorum; a tendency to produce such sori might be inherited 
by the descendants of that Asplenitum and augmented until these 
sori came to be produced not only on such veins as at first, but 
on other veins as well, and finally to supplant the asplenioid 
sori. 
