The microscope and its revelations . Campylodisctis costatus :—A, front view; b, side view. by its ribbed markings, which seem to indicate the presence ofcanals excavated in or beneath the valves, for the passage offluid between the orifices in the siliceous envelope and the softcell-membrane beneath. The form of the valves, in most of thespecies, is circular, or nearly so; some are nearly flat, whilstin others the twist is greater than in the species here repre-sented. Some of the species are marine, whilst others occurin fresh water; a very beautiful form, the C. clypeus, exists insuch abund
The microscope and its revelations . Campylodisctis costatus :—A, front view; b, side view. by its ribbed markings, which seem to indicate the presence ofcanals excavated in or beneath the valves, for the passage offluid between the orifices in the siliceous envelope and the softcell-membrane beneath. The form of the valves, in most of thespecies, is circular, or nearly so; some are nearly flat, whilstin others the twist is greater than in the species here repre-sented. Some of the species are marine, whilst others occurin fresh water; a very beautiful form, the C. clypeus, exists insuch abundance ia the Infusorial stratum discovered by at Soos near Ezer in Bohemia, that the earth seems * See Mr. BrigMweUs excellent memoir On the genus Trieeratium, in Quart. Microse. Joum., vol. i. p. 245. DlATOilACE^ :—SUKIRELLA, NAVICULA. 321 almost entirely composed of it.—Some of the forms of the lastgenus lead towards Surirella (Fig. 88), which, like it, presents Surirella constricta;—a, side view; b, front view; c. binaiy subdivision. the appearance of possessing a canalicular system, thoughthis is by no means equally conspicuous in all the distinctive character of the genus, in addition to thepresence of the canaliculi, is derived from the longitudinalline down the centre of each valve (a), and the prolongationof the margins into alse. Numerous species are known, whichare mostly of a somewhat ovate form, some being broader andothers narrower than S. consfricta; the greater part of themare inliabitants of fresh or brackish water, though some feware marine; and several occur in those infusorial earths, whichseem to have been deposited at the bottoms of lakes, such asthat of the Mourne mountains in Ireland (Fig. 102, b, c, k). 184, We now come to that interesting series of forms,which has been ranked under the genus Navicida, until itsrecent subdivision by Prof. W. Smith into the three generaNavicula, Pinmdaria, and Pleurosigma, e
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