[Illustration: FIG. 198.--_Frullania dilatata._ Portion of a branch seen from the under side: _r_ and _b_ are the anterior and posterior edges of the same dorsal leaf; _a_ ventral leaves (amphigastria). The dorsal leaves are “overlying,” _i.e._ the anterior edge of the leaf overlaps the posterior edge of the preceding one.] All the species in this family were formerly reckoned as belonging to one genus, _Jungermannia_, but now they are divided into several, arranged as follows:-- I. ANACROGYNÆ. The archegonia are situated on the upper side of the thallus or stem, _placed laterally_, and covered by an “involucre,” formed by the calyptra together with the tissue of the stem or thallus. a. ANELATEREÆ. Without any elaters: _Sphærocarpus_, _Riella_. b. ELATEREÆ. α. Thalloid: _Aneura pinguis_, in damp situations; _Metzgeria furcata_, on trees; _Pellia epiphylla_, in damp situations; _Blasia pusilla_, on damp clay soil, in the shade (scales are present on the thallus). β. Foliose and not dorsiventral: _Haplomitrium hookeri_. II. ACROGYNÆ. The apex of the stem or of certain branches is adapted for the formation of female shoots. The archegonia are most frequently aggregated on the apex of the shoots, and are encircled by their leaves (perichætium). Between these and the archegonia, enclosing the latter, a peculiar cup-shaped organ (the involucre) is formed. This group only includes leaf-bearing genera: _Frullania_, _Radula_, _Madotheca_, _Ptilidium_, _Calypogeia_, _Lepidozia_, _Mastigobryum_, _Lophocolea_, _Jungermannia_, _Scapania_, _Plagiochila_. Class 2. =Musci frondosi or veri (True Mosses).= In this class the protonema is well developed, and resembles a branched filamentous Alga, from which it can be easily distinguished by its oblique septa (in _Sphagnum_ it is a cellular expansion). The Moss-plant, which is developed directly from the protonema, generally has an erect, thick, cylindrical stem similarly constructed on all sides. The leaves are arranged spirally, the most frequent divergence being 2/5 or 3/8 (Fig. 200 _A_). A midrib is often present and also marginal veins formed by longitudinally elongated cells; at these veins the leaf is more than one layer in thickness. In _Leucobryum_ the leaves are generally constructed of more than one layer. The stem grows by means of a three-sided, pyramidal, apical cell which gives rise to three rows of segments, each segment forming a leaf. The lateral branches arise from the lower portions of the segments, the upper portion of which does not take any part in the construction of the leaf. From their mode of origin the branches are not axillary, and differ in this respect from the Flowering-plants.