The capsule opens as in _Sphagnum_ by means of a _lid_ (_operculum_), which is often prolonged into a beak. Round the mouth of the opened capsule, a number of peculiar yellow or red teeth are to be found. These constitute the _peristome_; their number is four, or a multiple of four (8, 16, 32 or 64). The form and thickenings of these teeth are widely different, and on this account are used by Systematists for the purposes of classification. In some Mosses (Fig. 200 _C_, _D_) there is a double row of teeth. Except in _Tetraphis_ they are not formed from entire cells, but from the strongly thickened portions of the wall of certain layers of cells belonging to the lid, and persist when this falls off. They are strongly hygroscopic, and assist greatly in the ejection of the lid, in which operation they are considerably aided by a ring of elastic cells with thickened walls, situated in the wall of the lid near the base of the teeth. This ring is known as the _annulus_. The archegonium is raised into the air like a hood, the calyptra, which either covers the sporangium on all sides (having the shape of a bell), or is split on one side (Fig. 200 _B_, _h_). Among peculiar forms may be mentioned: _Splachnum_, which is especially remarkable for the collar-like expansion at the base of the capsule. _Fissidens_ deviates in having a flat stem and leaves arranged in two rows. The leaves are boat-shaped and half embrace the stem.--_Schistostega_ has two kinds of stems. The barren ones resemble Fern-leaves; they have two rows of leaves, which are attached together vertically, are decurrent and coalesce at their bases. The fertile ones have an ordinary appearance.--_Tetraphis_: the peristome is composed of four teeth, which are formed from entire cells. _T. pellucida_ has peculiar gemmæ. The family is divided into two groups: the Musci acrocarpi, the growth of whose main axis is limited and terminated by the formation of the sexual organs; and the Musci pleurocarpi, whose sporogonia are situated on special lateral shoots, while the growth of the main axis is unlimited. [Illustration: FIG. 200.--_A Hypnum populeum_. _B_ and _C_ Sporangia, with hood (_h_), and operculum (_l’_), and without these (_C_), showing the peristome (_p_). _D_ The mouth of the capsule of _Fontinalis antipyretica_.] A. =Acrocarpi.= Order 1. =Weisiaceæ.= Peristome, with 16 teeth arranged in one series, rarely wanting. Leaf with midrib. _Campylopus_, _Dicranum_ (_D. scoparium_, common in forests), _Dicranella_, _Cynodontium_.--_Weisia_, _Gymnostomum_ (no peristome), _Systegium_. Order 2. =Leucobryaceæ.= Peristome with 16 teeth. Leaves with three or more layers of cells, of which the external ones are air-conducting and perforated (as in the Sphagneæ), the middle one containing chlorophyll. _Leucobryum._