_Bacillaria._ _Plate IV. fig. 6._--A simple siliceous frustule, of a prismatic shape, forming a brilliant chain, which often appears in zigzag, in consequence of incomplete self-division. An immense number and variety of forms are placed in this family by Ehrenberg, with a multitude of generic and specific names. The fresh-water species inhabit every pond and lake, and the marine every sea. Fossil species are equally abundant. _Cocconeis._ _Plate IV. fig. 7._--This is a very elegant type; the frustule consists of a simple siliceous case, with a central opening; it never occurs in chains like the former. It has been found fossil near Cassel. _Navicula._ _Plate IV. figs. 8, 9, 14, 15._--The plants of this genus are free, and float in the water apparently by the agency of cilia. Their case is a boat-like envelope with six openings, composed of pure silex, and in many species is exquisitely ornamented. Figs. 8 and 9. show a living _Navicula_, viewed in front, and in profile: in _fig. 9_ are represented the currents produced when the body is moving through the water; after Ehrenberg. Fossil _Naviculæ_ abound in many tertiary strata. _Galionella._ _Plate IV. figs. 10, 11._--These algæ are free, and the frustules of a cylindrical, globular, or discoidal form; they occur in chains, in consequence of the self-division being imperfect, and the new individuals remaining attached to the old. The _Galionellæ_ are most abundant and prolific, and inhabit every pool, stream, and lake: fossil species occur in the Virginian marls, and other strata. _Synhedra._ _Plate IV. fig. 12._--The frustules are siliceous, and of a slender, elongated form. The plant is attached by the base (_fig. 12 a._) in youth, and afterwards becomes free. It is found fossil in the Mountain-meal of Santa Flora, and many other deposits. _Podosphenia._ _Plate IV. fig. 13._--The frustule is cruciform, or wedge-shaped, and attached in youth by the small end, but afterwards becomes free. These plants are often arranged in clusters, as in the figure. M. Ehrenberg states that they inhabit the sea, and not fresh-water; but I have found them in streams communicating with the Thames. _Podospheniæ_ abound in the polishing slate of Bilin. _Eunotia._ _Plate IV. figs. 16, 17._--The frustule is siliceous, and either simple or bivalve; flat below, and convex, and often richly dentated above. An empty case is shown _fig. 16_; and a group of living _Eunotice_ attached to a stem of conferva, _fig. 17_. Several fossil species have been discovered at Santa Flora. That the general reader, whose attention is for the first time directed to this subject, may be prepared for the enormous deposits of fossil diatomaceæ that are found in some formations, I subjoin the observations of Dr. Bailey on an elegant fragile species, which hangs together in clusters, appearing like spiral chains, and is about 1/20 of a line in diameter; it is named _Meridion vernale_.