DIATOMA´CEÆ, or Siliceous Algæ.--The members of this family are singly very minute; but when existing in large numbers, as they are often found at the bottom of ditches and ponds, on the submerged stems of water-plants, or upon damp ground, they form yellowish-brown evident masses or strata. They occur both in sea-and in fresh water. They usually consist, like the Desmidiaceæ, of single cells, which are called frustules. But they are especially characterized by the cell-walls being imbued with silica or flint, so that if the frustules be heated to redness upon the point of a knife or a slip of platinum-foil, which destroys the organic part of the cells, the coat of silica remains, exhibiting the perfect form of the original cells or frustules. The form of the frustules is very different in the various genera and species, as represented in Pl. V. figs. 22, 23, 27, 30, 31, and Pl. VI. figs. 16, 17, 23; and it will be noticed that, in the figures, two views are given of each frustule, _f_ indicating the front view, and _s_ the side view. In all the front views, as in Pl. V. fig. 22, one or more lines will be observed running longitudinally down the middle of the frustules, and corresponding to the indications of division existing in the cells of the Desmidiaceæ. Each half of a frustule is called a _valve_, and the line at which these valves meet is called the _suture_. That side or aspect of the frustule in which the suture lies (fig. 22 _f_) is the _front view_; and the other aspect of the frustule (fig. 22 _s_) is the _side view_. The frustules are mostly four-sided--the main breadths of the two opposite valves forming two sides, and the bent margins of the valves, with the back and front of the hoop, forming the two other sides; so that the view presented by the side of a frustule is the same as that of a single valve. The suture is the line at which the division of the frustules takes place in the formation of new individuals. In this process the cell-contents divide into two parts, as in ordinary endogenous cell-formation,--the two new surfaces thus produced becoming coated with a new portion of cell-wall or valve, so that two frustules now occupy the place of the original one. At the same time a siliceous band, encircling the frustules at the line of suture, is formed to fill up the interval between the edges of the parent valves; this is the _hoop_ (Pl. V. fig. 22 _f_; Pl. VI. fig. 10 _f_), and beneath it lie the two newly formed valves. In many cases I believe that each half-frustule becomes coated with a new entire cell-wall, with its siliceous valves.