[Illustration: FIG. 32. _A_, _Lithothamnion mamillosum_ Gümb. (i) In section, (ii) surface view [after Gümbel. (i) × 320, (ii) nat. size]. _B_, _Sycidium melo_ Sandb. (i) Surface view, (ii) transverse section (after Deecke). _C_, _Bactryllium deplanatum_ Heer. (i) Surface view, (ii) section, showing the thick wall and hollow interior (after Heer). _D_, Calcareous pebble from a lake in Michigan. Rather less than nat. size (after Murray).] B. CHLOROPHYCEAE (GREEN ALGAE). Thallus unseptate, having the form of a vesicle or a variously branched coenocyte, which may or may not be encrusted with carbonate of lime, or of filaments composed of cells containing a single nucleus, or of cells in which more than one nucleus occurs; in other instances consisting of a plate of cells or a cell-mass. Asexual reproduction by zoospores and other reproductive cells; sexual reproduction by means of the conjugation of similar gametes or by the fertilisation of a typical egg-cell by a motile spermatozoid. This family of algae is represented at the present day by numerous and widely distributed marine and fresh-water genera, as well as by genera growing in moist air or as endophytes in the tissues of higher plants[270]. Seeing how very few fossil forms have been described which have any claim to inclusion in this subdivision of the Algae, it is unnecessary to enumerate or define the various families of the Chlorophyceae. It is true that many species have been figured as examples of different genera of green algae, but few of these possess any scientific value. There is, however, one division of the Chlorophyceae, the Siphoneae, which must be treated at some length on account of its importance from a palaeobotanical and geological point of view. _a._ SIPHONEAE. Thallus consisting of simple or branched cells very rarely divided by septa, and containing many nuclei. In certain genera the branches form a pseudoparenchymatous tissue by their repeated branching, and as a result of the intimate felting together of the branched cells. Reproduction is effected either by the conjugation of similar gametes or by the fertilisation of an egg-cell. _Vaucheria_ and _Botrydium_ are two well-known British genera of this order, but most of the recent representatives live in tropical and subtropical seas. The most striking characteristic feature of this division of the Chlorophyceae is the fact that the thallus of a siphoneous alga consists of an unseptate coenocyte; the plant may be extremely small and simple, or it may reach a length of several inches, but in all cases the body does not consist of more than one cell or coenocyte.