In the next group, the valves have a longitudinal line running down the middle of the valves, with a little knob or nodule in its centre ([Pl. V.] fig. 22 s), both consisting of internal thickened portions of the valves.
In Cocconéis placen´tula ([Pl. V.] fig. 33) the valves are oval, and the markings consist of longitudinal rows of minute dots, with a marginal row of puncta; these markings are invisible under ordinary illumination.
In Gomphonéma acuminátum ([Pl. V.] fig. 17) the frustules are attached to a branched stalk (stipes); they are wedge-shaped or cúneate (cúneus, a wedge) and transversely striate (fig. 18), the striæ consisting of dots.
In Pinnulária vir´idis ([Pl. V.] fig. 22), which is very often seen slowly traversing the field of the microscope when a drop of pond-water is examined, the frustules in the front view are linear, the valves being elliptic oblong, and transversely striated, the striæ consisting of furrows.
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.
In Coscinodis´cus radiátus ([Pl. VI.] fig. 3) the frustules are disk-shaped, the valves being elegantly sculptured with easily recognized cell-like markings or dots, so as to resemble a piece of vegetable cellular tissue. But in some other species the dots are very minute, and difficult to be shown satisfactorily. These markings consist of depressions or pits in the surface of the valves. That this is the case may easily be seen by examining a fragment of the valve, when the shadings of the broken ends of the netted thicker portions, which project like teeth, strongly contrast with the difficultly distinguishable portions of the thin interspaces. The fossil forms from the Bermuda deposit are best for the investigation of this structure; many of these are extremely beautiful microscopic objects, their markings resembling those on the engine-turned back of a watch.