In _Gyrosig´ma (Pleurosig´ma) attenuátum_ (Pl. VI. fig. 16) the valves are sigmoid, or somewhat resemble a Greek ς (sigma) in outline, and the markings consist of rectangularly crossed rows of very fine dots; in the front view, the frustules are linear-oblong with truncate ends. _Tabellária flocculósa_ (Pl. VI. fig. 23) has the frustules adherent only at the angles, as in _Diátoma_. They are rectangular, and in the front view exhibit a row of longitudinal dark lines interrupted in the middle; these have been compared to the vittæ of the fruit of the Umbelliferæ, and have received the same name. Among the marine species may be mentioned _Melosíra nummuloídes_ (Pl. VI. fig. 9), in which the frustules are united into a chain or cylindrical filament. This is very common among sea-weeds, &c.; and it illustrates well the process of division of the frustules (fig. 10 _f_). The valves are covered with fine dots, and near each end of the frustules is a projecting rim encircling it, and appearing as a curved line extending beyond the margin of the frustule in the front view. In _Actinocy´clus undulátus_ (Pl. VI. fig. 5) the frustules are separate, disk-shaped, and the valves are divided into six equal parts by six rays, each alternate portion of the surface of the valves being situated on a lower level than those adjacent, so that an alteration in the focus is required to bring into view the dots on any two adjacent divisions of the valve. The surface of the valves is covered with easily recognized dots. The form of the surface is best seen in the front view (fig. 5 _f_) when the frustule is placed on its edge. _Rhabdonéma arcuátum_ (Pl. VI. fig. 7), which is very commonly found attached to sea-weeds, resembles _Tabellária_ in the frustules having the vittæ (Pl. VI. fig. 8 _f_). The frustules form short filaments, attached by a little gelatinous cushion. The valves have transverse striæ, interrupted in the middle (fig. 8 _s_). _Gyrosig´ma (Pleurosig´ma) angulátum_ (Pl. VI. fig. 17) resembles _G. attenuátum_ in the sigmoid form; but the markings consist of lines crossing each other obliquely; and these are resolvable into rows of dots (fig. 17 _a_) under suitable illumination.