DIATOMACEiE : SURIRELLE.E | CAMPYLODISCUS. 



295 



Numerous species are known, which are mostly of a somewhat 

 ovate form, some being broader and others narrower than S. 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. 145, b, c, k). — In the Grenus Campylodiscus (Fig. 130) the 

 valves are so greatly increased in breadth as to present almost 

 the form of disks (a), and at the same time have more or less of 

 a peculiar twist or saddle -shaped curvature (b). It is in this genus 



Fig. 130. 



Campylodiscus costatus : — a, front view ; b, side view. 



that the supposed 'canaliculi' are most developed, and it is con- 

 sequently here that they may be best studied ; and of their being 

 here really costce or internally projecting ribs, no reasonable doubt 

 can remain after examination of them under the Binocular micro- 

 scope, especially with the Black -ground illumination. 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 C. chjpeus, exists 

 in such abundance in the Infusorial stratum discovered by Prof. 

 Ehrenberg at Soos near Ezer in Bohemia, that the earth seems 

 almost entirely composed of it. 



^ 226. The next Family, Striatellece, forms a very distinct group, 

 differentiated from every other by having longitudinal costoe on 

 the connecting portions of the frustules ; these costae being formed 

 by the inward projection of annular siliceous plates (which do not, 

 however, reach to the centre), so as to form septa dividing the 



