. The microscope and its revelations. ubercularelevations. Although the triangular formof the frustule, when looked at sideways, is that which is characteristic of the genus, yet in some of the speciesthere seems a tendency to produce quadrantjular \\\\<\ even pentagonalforms, these being marked as varieties by their exact correspondencein sculpture, colour, Arc., with the normal triangular Thisdeparture is extremely remarkable, since it breaks down what seemsat first to be the most distinctive character of the genus ; and itsoccurrence is an indication of the degree of latitude whi
. The microscope and its revelations. ubercularelevations. Although the triangular formof the frustule, when looked at sideways, is that which is characteristic of the genus, yet in some of the speciesthere seems a tendency to produce quadrantjular \\\\<\ even pentagonalforms, these being marked as varieties by their exact correspondencein sculpture, colour, Arc., with the normal triangular Thisdeparture is extremely remarkable, since it breaks down what seemsat first to be the most distinctive character of the genus ; and itsoccurrence is an indication of the degree of latitude which we oughtto allow in other cases. It is difficult, in fact, to distinguish thesquare forms of Friceratium from those included in the genus 1 See Mr. Brightwells excellent memoirs On the genus Triceratium inQuart. Jonrn. Microsc. Science, vol. i. 185!:!, p. 245 ; vol. iv. 1856, p. 272 ; vol. , p. 153; also Wallicli in the same Journal, vol. iv. 1858, p. 242 ; and Greville inTrans. Microsc. Soc. vol. ix. 1861, pp. 43, Fi<;. 457.—Isthmia nervosa. 614 MICROSCOPIC FORMS OF VEGETABLE LIFE—THALLOPHYTES AmpMtetras, which is chiefly characterised by the cubiform shape ofits frustules. In the latter the frustules cohere at their angles, soas to form zigzag filaments, whilst in the former the frustules areusually free, though they have occasionally been found in chains. Another group that seems allied to the Biddulphiece is the curiousassemblage of forms brought together in the family Chcetocerece, someof the filamentous types of which seem also allied to the peculiar distinction of this group consists in the presence oftubular awns, frequently proceeding from the connecting hoop,sometimes spinous and serrated, and often of great length (fig. 458);by the interlacing of which the frustules are united into filamentswhose continuity, however, is easily broken. In the genus Bacterias-truin (fig. 459) there are sometimes as many as twelve of these awns,
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