308 DIATOMACE/E : NAVICULE.E ; SCHIZONEME.E. 



being often extremely trivial ; those which have a lateral sigmoid 

 curvature (Fig. 120) have been separated by Prof. W. Smith under 

 the designation Pleurosigma, which is now generally adopted ; but 

 his separation of another set of species under the name Pinnularia 

 (which had been previously applied by Ehrenberg to designate the 

 striated species, those which seem to be smooth having been re- 

 ferred to Navicula), on the ground that its striae are not resolvable 

 into dots, and are so strongly marked (Fig. 145, h) as probably 

 to indicate internal costse like those of Surirella (Fig. 129), is 

 not considered valid by Mr. Ralfs, on the ground that in many 

 of the more minute species it is impossible to distinguish with 

 certainty between strise and costae. The multitudinous species 

 of the genus Navicula are for the most part inhabitants of Fresh 

 water ; and they constitute a large part of most of the so-called 

 ' 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 Navicular 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 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 

 Navicular, &c, 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 com- 

 paratively 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.* 



234. Of the members of the sub-family Schizonemece, consisting 

 of those Naviculece in which the frustules are united by a gela- 

 tinous envelope, some are remarkable for the great external resem- 

 blance they bear to acknowledged Algas. This is especially the 

 case with the Genus Schizonema, of which the gelatinous enve- 

 lope forms a regular tubular frond, more or less branched, and 



* For some very curious examples of the extent to which variation in 

 form, size, and distance in strise, may take-place in this group, among 

 individuals which must be accounted as of the same species, see the 

 Memoirs of Profs. W. Smith and W. Gregory already referred to (p. 287, 

 note). 



