ciently illustrate the structure of the skeleton of this tribe of plants, the most durable part of which is composed of a transparent horny material.

The skeletons of the next sub-order of the Diatomacea, the Cymbellea, are composed of organic matter intimately blended with silica, and indestructible both by heat and acids. All the members of this sub-order, like those of the Desmidieæ, are remarkable not only for the beauty of their form, but still more so for the extreme delicacy of their markings, some of which even defy the highest powers of the microscope to define, and on this account they are generally employed as testobjects. These are the minute bodies which constitute the Bacillaria, or stick-like animalcules of Ehrenberg, some of which inhabit fresh, others brackish water, but a few are exclusively marine. Many of the Diatomaceæ occur as single frustules, or cells, others are joined end to end, and form long, flattened ribbons ; some are connected by their corners, so as to resemble zig-zag chains, others again are attached to foreign bodies by means of a simple or branching stem, whilst a few genera are invested with a gelatinous envelope.

The Naviculæ, so abundant in our ditches, ponds and marshes, are, as their name implies, more or less boat-shaped, and of a green hue, in consequence of containing a number of granules of that colour. They possess the power of spontaneous mobility, and may be occasionally seen to glide slowly across the field of the microscope. In the living state the markings on their

surfaces are not very apparent, but after boiling in nitric acid, the green matter entirely disappears, and with the same magnifying power the surfaces are seen to be covered with delicate striæ, which, under the highest powers, and with careful management of the light, are readily resolved into cells, dots, or lines.

Since the employment of these objects as tests of the definition of microscopes, observers have differed greatly as to the nature of their markings: the striæ were regarded at first as lines, but recent investigation has shown that the lines are clearly made up of dots, and it is now disputed whether these dots are caused by elevations or depressions of the surface. In my opinion, all the coarser markings are of cellular form, and those having a dotted appearance, as shown at B, in Fig. 28, are due rather to pits, or depressions, than to elevations of the surface; they are not, however, foramina, there being a thin layer of silica beneath them. In order to satisfy myself of the true nature of the markings of many of the Diatomacea, I select a slide in which there are several species of the same genus, and examine carefully those specimens in which the markings are the coarsest, and therefore, most easily recognized; having made up my mind as to their nature, I then pursue the same mode of investigation with those having more delicate markings, but yet of the same character, and thus arrive at a definite conclusion.

LECTURE IV.

SKELETON OF DIATOMACEE.

HAVING in the last Lecture made a few remarks upon the Desmidie and Diatomacea, and the probable nature of the markings on the surfaces of the latter, I now proceed to consider the mode in which the silica and organic matter in the individual specimens, or frustules as they are termed by the botanist, are disposed.

Each frustule may be regarded as a cell, in which, occasionally, a nucleus and nucleolus are contained. By boiling in nitric acid, or by the action of fire, all the organic matter will be removed, and the silica left; but if hydrofluoric acid be employed instead of nitric, as was first done by Professor Baily, the silica will be dissolved, and a flexible internal membrane, probably composed of the same horn-like material as the skeleton

Fig. 24, occurs in considerable abundance in the form of zig-zag chains: each frustule consisting of a quadrangular plate, having its middle and two outer edges thicker than the other parts, so that in section, or when viewed end-ways, it presents the appearance shown at f. Other species of this genus vary considerably in shape, one in particular has frustules much longer than

B

A

FIG. 22.

they are broad, and is consequently named D. elongatum. A very interesting and beautiful species of Diatoma is the Meridion circulare, which in the fresh state consists of c a series of wedge-shaped frustules, as shown at D, Fig. 22, arranged in the form of circular bands, as at c; these, however, are not flat, but when perfect, as represented at B, have one end raised above the other; a better view, however, is obtained

A, frustules of Meridion circulare seen edgeways. B C, frustules disposed in a circular form. D, five frustules more highly magnified.

when the chains are thrown on one edge, they then present the flattened screw-like form represented by a. This species is met with in tolerable abundance in England and, also, according to Professor Bailey, " occurs in immense quantities in the mountain brooks around