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. The detection of the finer markings of the Diatomaceæ, which, according to my view, consist of depressions like those upon the valves of _Coscinodiscus_, is a matter of great difficulty to those who are unaccustomed to the use of the microscope, and who have not a complete set of apparatus. The main point to be attended to in bringing them into view, is to use one-sided oblique light, _i. e._ to turn the mirror by its stem as much as possible to one side, and then to incline it so as to throw the light upon the object. In this way the valves of the species of _Gyrosigma_, for instance, appear covered with lines (Pl. VI. figs. 16 and 17); but the lines are spurious, _i. e._ they are the optical expression of rows of minute dots (figs. 16 _a_, 17 _a_); and when oblique light is thrown upon the valves from all sides, by means of a special achromatic condenser, in which the central rays are excluded, the dots become distinct, and the markings resemble those on the valves of _Coscinodiscus_. To show the finer dots clearly, a valve should be crushed, so as to obtain a fragment as flat as possible; for the surface of the valves is curved more or less in all the species. The valves of _G. angulatum_ are generally used to test the quality of the object-glasses of the microscope, and also for practice in “making out” the lines and dots; there are, however, many Diatomaceæ more difficult. As the nature of these markings is a disputed point, the discussion of which is not adapted for an elementary work, I must refer for further details to the ‘Micrographic Dictionary;’ it may be remarked, however, that some observers have regarded them as cells, and others as elevations or tubercles on the surface of the valves. The preparation of the valves for showing the markings should be effected by burning the frustules, or the mass containing them, on a strip of platinum-foil over a spirit-lamp. The incinerated mass should then be transferred to a slide, and the valves separated with the greatest care by a bristle mounted in a hair-pencil stick under a low power of the microscope.