In _Fragilária capucína_ (Pl. V. fig. 23), which is extremely common in fresh-water pools, &c., the frustules are united side by side into long filaments, which are often twisted. In the separate and prepared frustule, the front view (Pl. V. fig. 24 _f_) is rectangular, the valves (_s_) being narrowly lance-shaped or lanceolate. The valves under ordinary illumination appear colourless and without markings, but, by proper management of the light, very fine transverse striæ are seen upon them, consisting of rows of very minute dots. Fig. 24 _s*_ represents the valve of _Fragilaria vires´cens_, a nearly allied species. _Diat´oma elongátum_ (Pl. V. fig. 31) is often found with the above. Its frustules are coherent at the angles. The front view (fig. 32 _f_) is rectangular, often slightly narrowed in the middle; and the valves are narrowly linear, and capitate at the ends; they are also transversely striated. In _Synédra splen´dens_ (Pl. V. fig. 5) the frustules radiate from a soft gelatinous cushion. They are linear in the front view (fig. 4 _f_), the valves (fig. 4 _s_) being gradually narrowed or attenuated from the middle to the ends, and exhibit transverse striæ interrupted opposite a middle longitudinal line. In _Campylodis´cus costátus_ (Pl. V. fig. 27) the frustules are disk-shaped and curved, so as somewhat to resemble a saddle. The markings consist of central dots, with radiating coarse flutings. _Nitzsch´ia minutis´sima_ (Pl. V. fig. 28) has oblique valves, _i. e._ the front half of the suture is not opposite the back portion; the valves (fig. 29) are constricted in the middle, and the ends narrowed and prolonged. The markings consist of a row of oblong dots or puncta (_punctum_, a point). This species often forms yellowish layers upon damp paths, &c. 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.