To this belong the majority of the Mosses, about 3,000 species.
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_.]