four new Brtish Lichens. 95 
elevation of it. Besides this, the crust of the two species is alto- 
gether dissimilar : in subfuscus it is continued, white, solid, be- 
tween leprous and granular; in porriginosus it is a congeries of very 
minute, powdery, greenish, unconnected particles, thinly spread 
over the bark of trees, and almost mealy. Between the present 
species and L. verncdis there is a still greater resemblance ; their 
crust is nearly the same, they seem to prefer the same tree, and 
even their shields differ in no particular so much as in colour ; 
which, however, among the Lichens, at least in the present state 
of our knowledge of them, must, if constant, be allowed to be suffi- 
cient to form the basis of specific characters. Relying upon this 
circumstance, there is no difficulty in discriminating between 
them at first sight ; especially by observing the borders of the 
young scutelloe, which are highly useful in distinguishing both 
these species, but to which, in L. xernalis, sufficient attention has 
not been paid. I am not aware that L. porriginosus is likely to be 
overlooked for any other British species : but in the foreign cata- 
logue there is one that it still more nearly resembles : this is 
L. viridesccm of Dr. Schrader's admirable Spicilegium Florce Ger- 
manica, from which its leading characters of difference are its 
powdery crust, the white margin of its young shields, their pre- 
serving in age a regular form, and their being thinly scattered, 
not crowded together, or often heaped upon each other, as some- 
times occurs in Mr. Dickson s L. Sphceroides. L. porriginosus may 
be reckoned among the more rare species, having, as far as I have 
heard, been hitherto found only in one place, and there merely 
on a few trees. I first met with it in company with Mr. Dickson in 
the beginning of April 1802. From its remarkable crust it may, 
as mentioned in the description, easily be overlooked as a variety 
of Byssus botri/oides, and the deception is considerably increased 
by its growing, like that species, chiefly near the roots of trees. 
IX. Descriptions 
