The Cambridge natural history . s. xiii. p. 71 ; xiv. p. 1 ; xvii. ]>. 93 ; J. , Trans. Micr. Hoc. i. p. 123 ; Ehrenbaum, Zeif. iriss. Zool. xli. \\ 1- DEPOSITION AND FORMATION OF SHELL 55 Formation of ^—The mantle margin is the principulagent in the deposition of shell. It is true that if the shell befractured at any point, the hole will he repaired, thus showingthat every part of the mantle is furnished with shell-depositingcells, hut siich new deposits are devoid of colour and of periostra-cuni, and no observation seems to have been made with regard tothe layers of whi
The Cambridge natural history . s. xiii. p. 71 ; xiv. p. 1 ; xvii. ]>. 93 ; J. , Trans. Micr. Hoc. i. p. 123 ; Ehrenbaum, Zeif. iriss. Zool. xli. \\ 1- DEPOSITION AND FORMATION OF SHELL 55 Formation of ^—The mantle margin is the principulagent in the deposition of shell. It is true that if the shell befractured at any point, the hole will he repaired, thus showingthat every part of the mantle is furnished with shell-depositingcells, hut siich new deposits are devoid of colour and of periostra-cuni, and no observation seems to have been made with regard tothe layers of which they are composed. As a rule the mantle,except at its margin, only serves to thicken the innermost layerof shell. It is probable that the carbonate of lime, of which the shellis mainly composed, is separated from the blood by the epithelialcells of the mantle margin, and takes the crystalline or granularform as it hardens on exposure after deposition. Tlie threelayers of a porcellanous shell are deposited successively, and. ABC Fig. 163.—Sections of shells. A, Conus : o, outer layer ; ft, middle prismatic layer, witliobliquely intersecting laminae above and below ; c, inner layer. B, Oliva : a, outerlayer; b, layer of crossed and curved laminae ; c, prismatic layer, succeeded bylayer of laminae at right angles to one another; d, inner layer. C, Ci/praea: a,outer layer ; b, middle layer ; c, inner layer. the extreme edge of the mouth, when shell is forming, willcontain only one lajer, the outermost; a little further in, twolayers appear, and further still, three. The pigment cells whichcolour the surface are situated at the front edge of the mantlemargin. Shelly matter is deposited, and probably secreted, not only bythe mantle, but also in some genera by the foot. This is certainlythe case in Cyml)turn, Olira, Ancillnria, Cassis, Distortio, andothers, in several of which tlie foot is so large that the shellappears to be quite immersed in 1 See also p. 258. - J. E. Gr
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