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DIATOMACE.K 617 



genus Xactcula and its allies the frustules are free or simply 

 adherent to each other ; while in another large section they are 

 included within a gelatinous envelope, or are enclosed in a defi- 

 nite tubular or gelatinous frond. Of the genus Xavicula an 

 immense number of species have been described, the grounds of 

 separation being often extremely trivial. Those which have a 

 lateral sigmoid curvature have been separated by Mr. W. Smith 

 under the designation Pleurosigma, which is now generally adopted ; 

 but his separation of another set of species under the name Piitnu- 

 laria (which had been previously applied by Ehrenberg to designate 

 the striated species), on the ground that its stria? (costa?) are not 

 resolvable into dots, was not considered valid by Mr. Ralfs, 

 because in many of the more minute species it is impossible to 

 distinguish with certainty between strife and costa 1 . Mr. Slack has 

 since given an account of the resolution of the so-called costa? of 

 twelve species of Pi/malar ice into ' beaded ' structures. 1 The beauti- 

 ful genus Stnnroneis. 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 stria?, which forms a cross 

 with the longitudinal band. The multitudinous species of the genus 

 Xaricnla 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 re- 

 markable 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 Navicidce and 

 rinnularicK. The Polierschiefer, or ' polishing slate,' of Bilm 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 aggrega- 

 tion of frustules of Savicuhe, ike., which have been consolidated by 

 heat. The species of Pleurosiyma, 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 fossilised state. Of 

 Stauroneis some species inhabit fresh water, while others are marine ; 

 and the former present themselves frequently in certain ' infusorial 

 earths.' 



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

 those Namculece in which the frustules are united by a gelatinous 

 envelope, some are remarkable for the great external resemblance 

 they bear to acknowledged alga 1 . This is especially the case with 

 the genus Schizonema, 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. -464), all these frustules 

 having arisen from the binary division of one individual. In the 

 1 Monthly Microscopical Journal, vol. vi., 1871, p. 71. 



