In _Unio_ the periostracal or uppermost layer is very thin; beneath this is a prismatic layer of no great depth, while the whole remainder of the shell is nacreous (Fig. 162 A). Many bivalves show traces of tubular structure, while in the Veneridae the formation and character of the layers approaches closely to that of the Gasteropoda. Further details may be gathered from Carpenter’s researches.[336] =Formation of Shell.=[337]--The mantle _margin_ is the principal agent in the deposition of shell. It is true that if the shell be fractured at any point, the hole will be repaired, thus showing that every part of the mantle is furnished with shell-depositing cells, but such new deposits are devoid of colour and of periostracum, and no observation seems to have been made with regard to the layers of which they are composed. As a rule the mantle, except at its margin, only serves to thicken the innermost layer of shell. It is probable that the carbonate of lime, of which the shell is mainly composed, is separated from the blood by the epithelial cells of the mantle margin, and takes the crystalline or granular form as it hardens on exposure after deposition. The three layers of a porcellanous shell are deposited successively, and the extreme edge of the mouth, when shell is forming, will contain only one layer, the outermost; a little further in, two layers appear, and further still, three. The pigment cells which colour the surface are situated at the front edge of the mantle margin. [Illustration: FIG. 163.--Sections of shells. =A=, _Conus_: _a_, outer layer; _b_, middle prismatic layer, with obliquely intersecting laminae above and below; _c_, inner layer. =B=, _Oliva_: _a_, outer layer; _b_, layer of crossed and curved laminae; _c_, prismatic layer, succeeded by layer of laminae at right angles to one another; _d_, inner layer. =C=, _Cypraea_: _a_, outer layer; _b_, middle layer; _c_, inner layer.] Shelly matter is deposited, and probably secreted, not only by the mantle, but also in some genera by the foot. This is certainly the case in _Cymbium_, _Oliva_, _Ancillaria_, _Cassis_, _Distortio_, and others, in several of which the foot is so large that the shell appears to be quite immersed in it.[338] The deposition of shell is not continuous. Rest periods occur, during which the function is dormant; these periods are marked off on the edge of the shell, and are known as _lines of growth_. In some cases (_Murex_, _Triton_, _Ranella_), the rest period is marked by a decisive thickening of the lip, which persists on the surface of the shell as what is called a _varix_ (see p. 263). [Illustration: FIG. 164.--_Murex tenuispina_ L., Ceylon. × ⅔.] [Illustration: FIG. 165.--_Neritina longispina_ Récl., Mauritius. (Operculum removed.)]