THE SHELL. 43 



like expansion on each side of the spire (xlvi, 69), showing a 

 very regular growth by periods of half-whorls. 



The accretion of surface during growth is not continuous but 

 is made by minute lajj-ers, around the margin of the aperture, 

 each extending a little beyond its predecessor, and the edges of 

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

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

 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 usually 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 ; thus the cone, which we have 

 seen partitioning ofi" its whorls against an enemy and thus 

 seriously incommoding itself for room, would under normal 

 circumstances acquire for itself additional accommodation by 

 absorbing awa}?- most of the thickness of the enclosed whorls or 

 partitions, and Cyprsea, Nerita and Auricula (i, 14', assist 

 themselves in the same manner so as to become eventually an 

 external shell only, with a single cavity. In species with 

 lengthened spire, this method is not so practicable, because 

 onl}^ a small portion of the worls are enclosed within suc- 

 ceeding ones. 



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

 it will be readily seen that details of sculpture as striae, sulca- 

 tions, ribs, nodes, spines, etc., result from similar ornamentation 

 of the applied mantle ; thus the spine of a Murex, if closelj^ 

 examined, will be found to have a longitudinal seam upon its 

 front face, showing that it has been formed by a corresponding 

 digitation of the mantle. It is scarcely necessary to pursue 

 this subject further at this time ; reference to the plates will 

 demonstrate this relationship of shell and animal. 



The following are the principal modifications of form in the 

 gastropod shell. 



A. Regularly spiral. 



a. Elongated, subulate, elevated. Terebra (Ivii, 11). 



b. Ttirrited, turriculated, babylonic ; an elongated shell 

 with the whorls angulated or shouldered on their upper 

 part. Turritella (Ixvii, 56). 



c. Cylindrical, pupiform. Cylindrella, Pupa (i, 12). 



