The Jungermanniaceæ, or Scale Mosses, in their lowest forms bear the same resemblance to Lichens that many of the other Liverworts do. These lichenoid forms are lobed, leaf-like masses, sometimes ribbed and sometimes not. There are forty genera of this group of plants, distributed amongst fifteen tribes, and exhibiting great variety of form and structure; but in external aspect they are so closely connected, that a graduated series may be traced from the flat-lobed frond to the higher forms of erect-stemmed foliaceous plants, approaching in size and structure to some of the smaller mosses. The higher groups have a distinct upright stem with symmetrical leaves, which leaves, however, in the lower genera, are merely fleshy fronds without veins, of numerous and sometimes grotesque forms, while in the higher, they are generally oval, and disposed in spiral imbricated rows on each side of the stem and branches, every leaf overlapping part of that which is adjacent to it like a row of tiles. They are disposed after two distinctly different plans. In one, the leaves are arranged in a spiral, which turns from left to right; consequently, the anterior border of each inferior leaf is covered by the posterior border of that immediately above it. This constitutes the succubous group of frondose Jungermannieæ, which embraces five tribes. In the second case, the spiral of the leaves turns from right to left, and the anterior border of each inferior leaf covers the posterior border of that immediately above it. This constitutes the incubous group embracing three tribes of the frondose Jungermannieæ. Besides the imbricated spiral leaves, there are small leaves alternating with them, and applied directly on the stem between them. The male fruit of the Scale Mosses consists of antheridia, which form ovoid masses of cells variously disposed; in each of these cells, there is a filiform spermatozoid coiled in a circle in active motion, which is maintained after leaving the cell. The female fruit consists of archegonia, containing embryo cells, which ultimately develope into sporangia; but the plants arrive at their full development before the archegonia appear. The same plant may produce more than one crop of these objects, which are occasionally abortive. The stalk of the sporangium is first surrounded by the sac of the archegonium, then by an involucre or circle of bracts, placed in a rosette of leaves. The sporangia contain cells which ultimately become spores mixed with long cells containing elaters, that is, elastic spiral filaments. When the fruit is ripe, the sporangia split into four equal parts, which form a cross on the top of their stems, and the elaters spring out of their cells, and disperse the spores.