DIATOMACB^ — SURIRELLA, NATICULA. 



293 



FtG. 88. 



the valves, for the passage of fluid between the orifices in the 

 siliceous envelope and the soft cell-membrane beneath. The 

 form of the valves, in most of the species, is circular, or nearly 

 so ; some are nearly flat, whilst in others the twist is greater than 

 in the species here represented. Some of the species are marine, 

 whilst others occur in fresh ^water; a very beautiful form, the 0. 

 clypeus, exists in such abundance in the Infusorial stratum dis- 

 covered by Prof. Ehrenberg at Soos near Ezer in Bohemia, that 

 the earth seems almost entirely composed of it. Some of the 

 forms of the last genus lead towards Surirella (Eig. 88), which, 

 like it, presents the appear- 

 ance of possessing a canali- 

 cular system, though this is 

 by no means equally con- 

 spicuous in all the species. 

 The distinctive character of 

 the genus, in addition to the 

 presence of the canaliculi, is 

 derived from the longitudi- 

 nal line down the centre of 

 each valve (a), and the pro- 

 longation of the margins into 

 " alse." ^Numerous species 

 are known, which are mostly 

 of a somewhat ovate form, 

 some being broader and 

 others narrower than 8. con- 



stricta ; the greater part of them are inhabitants of fresh or 

 brackish water, though some few are marine ; and several occur 

 in those infusorial earths, which seem to have been deposited at 

 the bottoms of lakes, such as that of the Mourne Mountains in 

 Ireland (Fig. 102, 5, c, h). 



184. We now come to that interesting series of forms, which 

 has been ranked under the genus Navicula, until its recent sub- 

 division by Prof. "W". Smith into the three genera Navicula, Pin- 

 nularia, and Pleurosigma, each of which still comprehends a very 

 large number of species. They are all distinguished by the ob- 

 long or lanceolate form of their valves, by the convexity of their 

 surmces, by the presence of a longitudinal line along the middle 

 line of each valve, dilating into nodules at the centre and extre- 

 mities, and by the more or less conspicuous marking of the 

 valves with transverse or oblique striae, which, under a sufficient 

 magnifying power, are resolvable into a regular areolation, re- 

 sembling, though on a very minute scale, that of the more coarsely 

 marked Diatoms (§ 175). The genus Navicula, as now constituted, 

 is distinguished by these striae, and by the appearance of rows of 

 circular dots which they present, when sufficiently magnified. 

 In Pinnularia (Fig. 102, h), the strise are not resolvable into dots, 

 and are so strongly marked as almost to resemble the " ribs" of 



Surirdla consiricUi : — a, side view ; b, front view; 

 0, binary subdivision. 



