four new Brlish 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 porriginostts it is a congeries of very 

 minute, powdery, greenish, unconnected pailicles, thinly spread 

 over the bark of trees, and almost mealy. Between the present 

 species and L. vernalis 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 scutellae, which are highly useful in distinguishing both 

 these species, but to which, in L. vernalis, sufficient attention has 

 not been paid. I am not aware that L. porriginostts 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. viridescens of Dr. Schrader's admirable Spicilegium Florae. Ger- 

 manics, 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. porriginostts 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 Bt/ssus botryoides, and the deception is considerably increased 

 by its growing, like that species, chiefly near the roots of trees. 



JX. Descriptions 



