With regard to these crustaceous mosses in general, it is highly worthy our regard, that in the œconomy 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 estimation 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 corrupting, 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[105]. 5. Lichenes foliacei scutellati. _Such as consist of a more lax and flexible matter, formed into a foliaceous appearance, having the parts of fructification in the form of_ scutellæ. 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 second order of _lichenoides_ in Dillenius; the first division of the second series, and the latter part of the second division, of the same: it comprehends the _lichenes imbricati_ and _umbilicati_ of Linnæus; 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 Linnæus’s observation, that one of the crustaceous lichens was scarcely ever found growing but upon limestone rocks. On the contrary, the same author has observed of a foliose lichen belonging to this order, that it will thrive on all kind of rocks but limestone rocks. This species[106] Dillenius calls _The common grey-blue pitted lichenoides_. It is very common 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_. Linnæus adds, that it will dye a purplish colour.