288 



THE MICROSCOPE AND ITS REVELATIONS. 



Campylodiscus costatus;A, front view; B, side view 



prolongation of the margins into 'alae.' Numerous species are known, 

 which are mostly of a somewhat ovate form, some being broader and 

 others narrower than 8. constricta; the greater part of them are inhab- 

 itants of fresh or brackish water, though some few are marine; and 

 several occur in those Infusorial earths which seem to have been deposited 

 &t the bottoms of lakes, such as that of the Mourne mountains in Ireland 



(Fig. 192, fl, c, &). In 



A "Etoov 37fo 15 the genus Campylodiscus 



(Fig. 176) the valves are 

 so greatly increased in 

 breadth as to present al- 

 most the form of disks 

 (A), and at the same time 

 nave more or less of a 

 peculiar twist or saddle- 

 shaped curvature (B). It 

 is in this genus that the 

 supposed 'canaliculi' are 

 most developed, and it is 

 consequently here that 

 they may be best studied; 

 and of their being here really costce or internally projecting ribs, no rea- 

 sonable doubt can remain after examination of them under the Binocu- 

 lar microscope, especially with the ' blackground ' 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. Soine of the species are marine, whilst others occur in fresh 

 water; a very beautiful form, the C. dypeus, 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. 



287. The next Family, Striaiellcw, forms a very distinct group, differ- 

 entiated from every other by having longitudinal costse on the connecting 

 portions of the frustules; these costae being formed by the inward pro- 

 jection of annular siliceous plates (which do not, however, reach to the 

 centre), so as to form septa dividing the cavity of the cell into imper- 

 fectly-separated chambers. In some instances these annular septa are 

 only formed during the production of the valves in the act of self-divi- 

 sion, and on each repetition of such production being thus always defi- 

 nite in number, whilst in other cases the formation of the septa is con- 

 tinued after the production of the valves, and is repeated an uncertain 

 number of times before the recurrence of a new valve-production, so 

 that the annuli are indefinite in number. In the curious Grammato- 

 phora serpent ina (Fig. 174) the septa have several undulations and in- 

 curved ends, so as to form serpentine curves, the number of which seems 

 to vary with the length of the frustule. The lateral surfaces of the 

 valves in Grammatopkora are very finely striated; and some species, as 

 G. subtilisswia and G. marina are used as Test-object ( 161). The 

 frustules in most of the genera of this family separate into zigzag chains, 

 as in Diatoma; but in a few instances they cohere into a filament, and 

 still more rarely are furnished with a stipes. The small Family Terpsi- 

 rioece is separated by Mr. Ralfs from the Striatelleae, with which it is 

 nearly allied in general characters, because its septa (which in the latter 

 are nearly longitudinal and divide the central portions into chambers) 

 are transverse and are confined to the lateral portions of the frustules, 



