The peculiarities of these plants are so remarkable and interesting that they deserve more than a passing notice. As a rule, to which, however, there are a good many exceptions, they do not grow upright in tufts like the mosses, but have a flat, creeping, lichen-like habit, spreading over rocks and trees in closely applied circles which radiate from a common centre. The whole typical plant is like a series or necklace of roundish, flat, imbricated scales, several of which branch from a common point in the middle. The leaves, unlike those of the mosses, are entirely destitute of a central nerve, for what is called the nervure in the membraneous or leafy species is nothing more than the stalk itself on the edges of which the leaves are fastened together in such a manner as to form apparently a continuous whole. The Hepaticæ, or scale-mosses, may be divided into two groups, consisting of those species in which the vegetation is frondose, that is, in which leaf and stem are confounded, and of those in which the vegetation is foliaceous, that is, in which leaves and stem are distinct. The most interesting of all the frondose group of scale-mosses is the common Marchantia or Liverwort (Marchantia polymorpha). It is very common, creeping in large, dark-green patches over rocks in very moist and shady situations, such as the banks of a densely wooded stream in a deep, narrow glen, or the sides of rivers and fountains. It may often be seen also on the moist walls of hothouses and in the pots and tubs. It adheres closely to rocks, which it sometimes completely covers with its imbricated fronds by the numerous white, downy radicles with which the under surface is covered. The second or foliaceous group of scale-mosses, in which the leaves and stem are distinct, is called Jungermanniæ, and contains by far the largest number of species and the richest variety of form and color. On either side of the thread-like stem arise in a more or less oblique position the membraneous overlapping leaves; while the fruit-vessel springs from the end of the stem, and is produced upon little silvery foot-stalks. It bursts into four valves, and when fully expanded spreads out into the form of a cross. There is a class of plants whose external appearance and mode of growth would indicate that they belong to the tribe under review, but whose structure and functions are so different that they are commonly supposed to bear a closer analogy to the ferns. They occupy an intermediate position, and form a connecting link between ferns and mosses; I allude to the Lycopods, or club-mosses. They are usually found in bleak, bare, exposed situations in all parts of the world, and sometimes attain a large size; forsaking the creeping habit peculiar to the family, and becoming slightly arborescent in tropical countries, particularly New Zealand, rivaling in rank luxuriance the smaller shrubs of the forest.