MICROSCOPIC FORMS OF VEGETABLE LIFE 



299 



largely used in the arts for the polishing of metals, under the names of 

 Tripoli and rotten-stone; these consist in great part of the frustules of 

 Naviculae and Pinnulariae. The Polierschiefer or 'polishing slate' of 

 Bilin in Bohemia, the powder of which is largely used in Germany for 

 the same purpose, and which also furnishes the fine sand used for the 

 most delicate castings in iron, occurs in a series of beds averaging four- 

 teen feet in thickness; and these present appearances which indicate that 

 they have been at some time exposed to a high temperature. The well- 

 known ' Turkey stone/ so generally employed for the sharpening of 

 edge-tools, seems to be essentially composed of a similar aggregation of 

 frustules of Naviculae, etc., which has been consolidated by heat. The 

 species of Pleurosigma, on the other hand, are for the most part either 

 marine or are inhabitants of brackish water; and they comparatively 

 seldom present themselves in a fossilized state. Of Stauroneis, some 

 species inhabit fresh water, while others are marine; and the former pre- 

 sent themselves frequently in certain ' infusorial earths.' 



ric. 188. 



Schizonema Grevillii : A, natural size; B, portion magnified five diameters; c, filament magni- 

 fied 100 diameters ; D, single f rustule. 



297. Of the members of the Suo-family Scliizonemew, consisting of 

 those Naviculem in which the frustules are united by a gelatinous enve- 

 lope, some are remarkable for the great external resemblance they bear to 

 acknowledged Algse. This is especially the case with the genus Schizo- 

 nema ; in which the gelatinous envelope forms a regular tubular frond, 

 more or less branched, and of nearly equal diameter throughout, within 

 which the frustules lie either in single file or without any definite 

 arrangement (Fig. 188); all these frustules having arisen from the self- 

 division of one individual. In the genus Mastogloia, which is specially 

 distinguished by having the annulus furnished with internal costae 

 projecting into the cavity of the frustule, each frustule is separately sup- 



