The microscope and its revelations . Actinocyclus tmdidatus :—A, side view;B, front view. DISCOIDAL DIATO^klACE^ :—HELIOPELTA. 317 but which are so precisely alike in every other particular, thatthey can scarcely be accounted as specifically different. Thevalves of this genus are very abundant in the infusorial earthsof Richmond, Bermuda, and Oran (Eig. 101, b, b, b); andmany of the same species have been found recent in guano,and in the seas of various parts of the world. The frustulesin their living state appear to be generally attached to sea-weeds or zoophytes, 182. The Bermuda earth also

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The microscope and its revelations . Actinocyclus tmdidatus :—A, side view;B, front view. DISCOIDAL DIATO^klACE^ :—HELIOPELTA. 317 but which are so precisely alike in every other particular, thatthey can scarcely be accounted as specifically different. Thevalves of this genus are very abundant in the infusorial earthsof Richmond, Bermuda, and Oran (Eig. 101, b, b, b); andmany of the same species have been found recent in guano, and in the seas of various parts of the world. The frustulesin their living state appear to be generally attached to sea-weeds or zoophytes, 182. The Bermuda earth also contains the very beautifulform (Eig. 85), which, though scarcely separable from Fig, 85.. Heliopelta, Actuiocyclus except by its marginal spines, has received fromProf. Ehrenberg the distinctive appellation of Heliopelta (sun-shield). In the representation here given, the object is deli-neated as seen by the parabolic illuminator (§ 61), whichbrings uito view certain features that can scarcely be seen by 318 MICEOSCOPIC POEMS OF YEGETAELE LIFE. ordinary transmitted light. Eour of the radial divisions areseen to be marked-ont into circular areolae; but in the fourwhich alternate with them, a minute granular structure isobservable. This may be shown by careful adjustment of thefocus, to exist over the whole of the valve, even on the divi-sions in wliich the circular areolation is here displayed; and ithence appears that this marking is superficial, and that thecircular areolation exists in a deeper layer of the siliceouslorica. In the alternating divisions whose surface is here dis-played, the subjacent areolation, when brought into view byfocussing down to it, is seen to b