The Pyxinei are horizontal foliaceous lichens, for the most part fixed by the centre. They have orbicular discs, and form one of the most singular groups, both with regard to the superficial fruit, and the curious convolutions of the perithecia. The fruit-cup, or excipulum, is at first closed, and in the genus Gyrophora the disc produces a number of partial fructiferous discs on the original fruit-bearing surface. As usual the cells from whence the asci spring belong to the medullary stratum; the border on the contrary is cortical. The thallus is always foliaceous, and more or less peltate, but it becomes dry and brittle when exposed to drought. Several of the species have tufts of strong rootlets by which the frond is fixed to rocks, and some have deep pits, with corresponding swellings on the upper-surface, from whence they have their popular name of Tripe de Roche. These lichens, together with some species of Gyrophora, afforded a miserable sustenance to Sir George Back and his companions during their journey along the frozen regions of Arctic America in quest of the north-west passage. It is evident that plants which derive their sustenance solely from the medium in which they live, whether air or water, can never become a permanent and wholesome food for man, though erratic lichens and laver may be eaten from necessity, certainly not from choice. In the Gyrophora the perithecia are convolute, in the Umbilicaria they are not; but both of these genera, which have their seat in Arctic Europe and America, possess fertilizing particles like most of their class.
In the order Graphidei the disc is linear, simple, or branched, with or without an excipulum, which is carbonaceous. Many of these lichens are crustaceous, and in most of the genera the perithecia are much elongated, pointed at both ends. When they are parallel, or placed at different angles to each other, they form groups like Japanese or Chinese characters, whence their name. The genera are determined by the position of these perithecia, which are much varied. M. Tulasne has found linear male particles immersed in the crust of some of these lichens. They have their principal seat in tropical America, though a few species occur in the frigid and temperate zones of both hemispheres.
