22 SHELL. 



these layers as exposed on the external surface are called growth- 

 lines. Those shells which have a simple, or sharp-lipped aper- 

 ture and which do not develop varices, nevertheless distinctly 

 show the rest-periods by the greater impression of the growth- 

 lines. Many shells retain the sharp aperture for a variable period, 

 which may be called their juvenescence, but finally acquire adult 

 characters, consisting of a thickened, reflected, inflected or lipped 

 aperture which is sometimes more or less contracted by inflected 

 calcareous projections called teeth. Growth, however, frequently 

 continues after adult characters have been acquired, and then 

 these are absorbed away when accretion recommences, leaving 

 the mark of their former position in a more prominent growth- 

 scar or line. 



The power of dissolving their shells is possessed by certainly 

 a large portion of the mollusca and is habitually exercised by 

 many of them : thus the cone, which we have seen partitioning 

 off its whorls against an enemy and thus seriously incommoding 

 itself for room, would under normal circumstances acquire for 

 itself additional accommodation by absorbing away most of the 

 thickness of the enclosed whorls or partitions, and Cypraea, Nerita 

 and Auricula assist themselves in the same manner so as to 

 become eventually an external shell only, with a single cavity. 

 Tn species with lengthened spire, this method is not so practi- 

 cable, because only a small portion of the whorls are enclosed within 

 succeeding ones. The growth of many of these animals is such 

 that they cease after awhile to occupy some of the earlier whorls? 

 and they then partition them off as a regular habit, in the same 

 manner that the cone has worked under the pressure of excep- 

 tional necessity. These partitions made, the portion thus cut off' 

 from contact with the animal loses vitality, becomes brittle and 

 breaks off, forming the truncated shells which are characteristic 

 of numerous groups especially of land shells. 



From what has been said of the mode of formation of shells it 

 will be readily seen that details of sculpture as striae, sulcations, 

 ribs, nodes, spines, etc., result from similar ornamentation of the 

 applied mantle : thus the spine of a Murex, if closety examined, 

 will be found to have a longitudinal seam upon its front face, 

 showing that it has formed by a corresponding digitation of 

 the mantle. It is scarcely necessary to pursue this subject 



