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and as long as the mollusc is advancing toward its full 

 growth, the mantle continues to deposit fresh materials 

 round the aperture, so that the shell grows with it, and 

 hence the striated structure, which, in some shells, is very 

 strongly marked, though obliterated in others. The mantle 

 deposits not only the substantial materials of the shell, but 

 also the epidermis, or outer coating, together with the 

 colours, patterns, and polished enamel with which many of 

 them are ornamented, and the internal lining which is 

 sometimes iridescent, as in the Haliotis, sometimes pearly 

 and lustrous, also the pearls which are accidents formed 

 within the shells of the pearl oysters and mussels. These 

 functions of the mantle are exercised more energetically on 

 one side than on the other, and hence the complicated forms 

 of the shells. It is obvious that if the minute shelly cup 

 pointed or rounded, which covers the mollusc in the first 

 stage of its existence, were added to equally and uniformly 

 round its edge, the growth of the shell would be straight- 

 forward, and the result would be a simple cone more acute 

 or obause according as the spread of the mollusc were more 

 or less rapid. This is precisely what happens in the case 

 of the Orthoceras, one of the extinct Cephalopoda, which is 

 a very long cone, and of the common Limpet, which is a 

 very short <me. Now, if we suppose the additions of the 

 shelly material to be mado on ouo side of the aperture more 

 rapidly than on the other, that side of the cone will grow 

 longer than the other, if at the same time the aperture is 

 kept square with the axis of the cone, the result will be a 

 curved form like the Foxoceras, another of the extinct 

 Cephalopoda, and such is the Dentalium, a Gasteropod. If 

 the addition of the material to one side of the aperture is so 

 rapid that one side of the shell overtakes the other, the shell 

 will be coiled as in the case of the Ammonite, which is a 

 cone precisely analogous to the Orthoceras and Toxoceras 

 rolled up. The structure of the Nautilus is the same, but 

 less conspicuously so than in the Ammonite, and this is the 

 character of all the testaceous Cephalopoda. Here you will 



