The Usneæ are perhaps the most beautiful of the lichens, the colours being sometimes brilliant, the forms elegant, and when the broad discs are amply ciliated, the appearance is very striking. The same species are widely diffused, but the colours are brighter in exotic specimens. Usnea melaxantha and Usnea Taylori are splendid productions.[63] The Usneæ are cosmopolitan in genera and species. The genus Cetraria, of which the Iceland moss is a well known species, forms a connection between the vertical and horizontal lichens; its thallus is neither cylindrical nor quite erect, though it becomes more so towards maturity. The Cetraria tristis has only that degree of inclination which arises from its crowded mode of growth, and springs like a sea-weed from a little peltate disc. The most typical species of lichens occur in the second group of Euparmeliaceæ, or Parmeliacei proper, in which the disc is at first closed, and surrounded after expansion by a border arising from the thallus or frond. The thallus is always horizontal, and expands from the centre towards the circumference. The genus Sticta, belonging to this group, is often highly foliaceous, and is not excelled by any horizontal lichen in brightness of colouring or elegance of form. Even in our own country the Sticta pulmonacea spreads over a wide area, and is remarkable for its pitted frond. In this genus the under-side of the plant is covered with a delicate velvety down, amidst which are scattered round white spots, which penetrate to the medullary strata. The erratic lichens are among the peculiarities of the genus Parmelia. The Parmelia saxatilis, common on stones and boulders of the primary and metamorphic formation, curls up into a ball, only fixed to its matrix by a slender thread, which soon gives way, and the ball being dry and as light as a balloon, is driven bounding by the winds, over the sheep walks and downs of England. The globular Lecanora esculenta sometimes suddenly covers large tracts in Armenia, Persia, and Tartary, where the plants are eaten by the cattle and by the nomade tribes. This species, with Lecanora affinis, is largely used by the inhabitants of the countries east of the Levant, and in some parts of Africa is mixed with wheat in grinding. These species are found, scattered over the ground without any attachment, in the form of rugged truffle-like objects of the size of walnuts. Sometimes they are piled together in strata a few inches thick, by the whirlwinds, after traversing the air for many miles, which gives rise to the histories of the miraculous descent of food. During a scarcity, a shower of these lichens fell at Erzeroum, and there are other well-authenticated instances recorded.[64]