. 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 the con lained prisms without a
. 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 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 , when a sufficiently high magnifying po\\er 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
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