294 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



FIG. 89. 



Surirella. The genus Pleurosigma is at once distinguished by 

 the peculiar curvature of its valves (Fig. 80) ; and it is further 

 remarkable for the extreme closeness of their striation, and for 

 the consequent "difficulty" which attends its resolution into a 

 regular areolation. The species of the first two of these genera 

 are for the most part inhabitants of fresh water ; and they con- 

 stitute a large part of most of the Infusorial Earths which were 

 deposited at the bottoms of lakes. Among the most remarkable 

 of such deposits, are the substances 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 ^aviculse 

 and Pinnularise. 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 fourteen feet in thickness ; and these present appear- 

 ances which indicate that they 

 have been at some time exposed 

 to a high temperature. The well- 

 known Turkey-stone, so gene- 

 rally employed for the sharpen- 

 ing of edge tools, seems to be 

 essentially composed of .a similar 

 aggregation of frustules of !N"a- 

 viculse, &c., which has been con- 

 solidated 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 compa- 

 ratively seldom present themselves 

 in a fossilized state. The genus 

 Stauroneis, which belongs to the 

 same group, differs from all the 

 preceding forms, in having the 

 central nodule of each valve 

 dilated laterally into a band free 

 from strise, which forms a cross 

 with the longitudinal band; of 

 this very beautiful form, some 

 species are fresh water, others 

 marine; and the former present 

 themselves frequently in certain 

 infusorial earths. 1 



185. The group we have next 

 to notice, consists of those genera which have the frustules, 



1 For some very curious examples of the extent to which variation in form, size, and 

 distance of striae, may take place in this group, among individuals which must be ac- 

 counted as of the same species, see the memoirs of Profrs. W. Smith and W. Gregory 

 already referred to (p. 287, note). 



Gomphonema geminatum : its frustules con- 

 nected by stipes. 



