256 PHILOSOPHICAL TRANSACTIONS. [anNO J 758. 



With regard to these crustaceous mosses in general, it is worthy oar regard, 

 that in the economy of nature they answer singular and important uses. To an 

 unobserving eye, no class of vegetables may appear more insignificant, or less 

 adapted to advantageous purposes in the creation than these. This vulgar esti- 

 mation of things is frequently erroneous ; and it is certainly so in the instance 

 before us. These minute and seemingly insignificant mosses serve, under some 

 circumstances, to valuable purposes. No sooner is a rock left bare by the sea, 

 but these lichens lay the foundation for its future fertility. Their seeds, which 

 are presently brought thither by the winds, soon cover it all over. These cor- 

 rupting, presently afford a soil sufficient to nourish other smaller mosses ; which 

 in their turn form one deep enough for larger plants and trees ; and thus the 

 rock becomes a fertile island.* 



5. Lichenes Foliacei Scutellati. 

 Such as consist of a more lax and flexible matter, formed into a foliaceous ap- 

 pearance, having the parts of fructification in the form of scutellae. 



Some of the plants of this division are interspersed with the former in some of 

 the systems of botanic authors. In general this division contains the whole first 

 series of the 2d order of lichenoides in Dillenius ; the first division of the 2d 

 series, and the latter part of the 2d division, of the same : it comprehends the 

 lichenes imbricati and umbilicati of Linneus ; and many of the placodium of 

 Hill. The plants of this order are many of them not less common in England 

 than the foregoing, on rocks, stones, old pales, trees, &c. Some adhere very 

 closely to what they grow upon, and seem to be only foliaceous about the edges : 

 others adhere but loosely, and are much expanded and divaricated, so as to form 

 something like ramifications. 



It was remarked, from Linneus's observation, that one of the crustaceous 

 lichens was scarcely ever found growing but upon limestone rocks. On the con- 

 trary, the same author has observed of a foliose lichen belonging to this order 

 that it will thrive on all kinds of rocks except limestone rocks. This species -f- 

 Dillenius calls the common grey-blue pitted lichenoides. It is very cx)mmon with 

 us upon trees, old wooden pales, &c. as well as upon rocks and stones. It is 

 the usnea cranii humani of the old materia medica. Linneus adds, that it will 

 dye a purplish colour. 



Hither likewise must be referred the cork or arcel,;}: which is used by the 



• Vide Econom. Natur. in Araaen. Acad. vol. ii. p. 17. — Orig. 



f Lichenoides vulgatissimum cinereo-glaucum lacunosum et cirrhosum Hist. Muse. p. 8S. Li- 

 chenoides crusta foliosa superne cinereo-glauca, infeme nigra et cirrbosa scutellis nigricantibus. R. 

 u Syn. p. 72. — Orig. 



X Lichenoides saxatile tinctorium foliis pilosis purpureis Raii Syn. p. 74, No. 70. Hist. Muse. p. 

 IS.*). Lichen petraeus putpureus Derbiensis Park. I heat p. 1315. Lichen ompbalodes Lin. Spec. 

 PI. 1143.— Orig. 



