. il microscopio e le sue rivelazioni. el di denti, towhich questo tipo di struttura a guscio può essere considerato come recanti una somiglianza molto vicina, tranne per quanto riguarda l'ingrediente mineralizzante. Se una sezione simile viene de-calcificata mediante acido diluito, il residuo rambranoso presenterà la stessa resemblanza alle pareti delle cellule pris-matiche viste longitu-dinallv. ;md sarà visto tolie più o meno regolarlymarchiato dalla transversestria? ha appena alluso a. A volte succede in re-cent ma ancora più com-mouny iu gusci fossili, that1 mentire decadimento della membrana animalleave.x il con lained prismi senza un

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. The microscope and its revelations. el of teeth, towhich this kind of shell-structure may be considered as bearing avery close resemblance, except as regards the mineralising ingredient. If a similar section be de-calcified by dilute acid, themembranous residuum willexhibit the same resem-blance to the walls of pris-matic cells viewed longitu-dinallv. ;md will be seen tolie more or less regularlymarked by the transversestria? just alluded to. Itsometimes happens in re-cent but still more com-monly iu fossil shells, that1 lie decay of the animalmembrane leave.x the con lained prisms without any connecting medium: as they are thenquite isolated, they can be readily detached one from another: andeach one may be observed to lie marked by the like striations.which, when a sufficiently high magnifying poer is used, are seento be minute grooves, apparently resulting from a thickening of theintermediate wall in those situations. These appearances >eem l>estaccounted for by supposing that each is lengthened by successive. FK;. 694.—Membranous basis of the same. STKUCTUEE OF SHELLS 921 additions at its base, the lines of junction of which correspond withthe transverse striation ; and this view corresponds well with thefact that the shell-membrane not unfrequently shows a tendency tosplit into thin lamina1 along the lines of striation. whilst we occa-sionally meet with an excessively thin natural lamina lying betweenthe thicker prismatic layers, withone of which it would haveprobably coalesced but for someaccidental cause which preservedits distinctness. That the prismsare not formed in their entirelength at once, but that they areprogressively lengthened andconsolidated at their lower ex-tremities, would appear alsofrom the fact that where the shell presents a deep colour (as Df . . L . FIG. 695.—Section of the shell of Pinna in / tuna mgmna) this colour in the direction of its prisms. is usually disposed in distinct strata, the outer portion of each layer being

Parole chiave associate all'immagine

• bookcentury1900
• bookdecade1900
• booksubjectmicrosc
• bookyear1901