Still another section of this large genus contains weeds of a more shrubby growth, with veined stem and branches jointed obscurely. Of these the Rosy Callithamnion (_C. roseum_) is not uncommonly found on muddy shores, and especially in and near the estuaries of rivers. It grows in dense dark-coloured tufts, two or three inches long, with alternate branches much divided. The tetraspores occur singly, one at the top of each of the lower joints of the pinnules of the plumes. _C. byssoideum_ grows on larger weeds in the rock pools, and especially on _Codium tomentosum_ (p. 353), in dense tufts of exceedingly fine filaments, jointed, and branched irregularly. The upper branches are plumed, and their tips bear very fine colourless filaments. The spore-clusters are arranged in pairs, and the tetraspores are thinly scattered on the pinnules of the plumes. This species is so very delicate in structure that a lens is absolutely necessary to make out its structure. It is, in fact, impossible to distinguish between the various species of Callithamnion without such aid; and many of them, particularly the species last described, require the low power of a compound microscope. Among the other common species, belonging to the same section, we may mention _C. corymbosum_, distinguished by its very slender, rosy, jointed fronds, with the ultimate divisions of the branches disposed in a level-topped (_corymbose_) manner, growing on rocks and weeds near low-water mark; _C. polyspermum_, growing in globular tufts on _Fucus serratus_ and _F. vesiculosus_, with short awl-shaped pinnules, and closely-packed clusters of spores; _C. Hookeri_, with opaque stem and branches, and spreading branchlets that are themselves branched, and bear spreading plumules at their tips; and _C. arbuscula_, found on the west coasts, with a stout stem, naked below, and having a very bushy habit. It is often by no means an easy matter to distinguish between the different species in such a large genus as _Callithamnion_, and we strongly recommend the beginner to first study the characteristics on which the classification of the _Algæ_ is based, and to arrange his specimens according to the orders and genera to which they belong; and then, after mastering the principles of classification, he should refer to one of those larger works in which all known British species are described, and make himself acquainted with the features of each individual species in his collection.