33 



see that the progress of the shell has always been forward 

 in the direction of the axis with which the aperture is 

 symmetrical. For the Gasterpoda the case is diiferent, with 

 some exceptions, such as we have seen in the Limpet and 

 Dentalium— the deposition of material by the mantle, that 

 is transverse in a sidelong direction to the axis of the shell, 

 and the result is a spiral form, the mouth of the shell 

 opening sideways, and, as a general rule, on the right side, 

 as it is regarded in front. This is the general character of 

 the Gasteropoda, in most of which the spiral form is ob- 

 vious, in some less obvious and in some very obscure. 

 Here is the terebra for example, forming a regular 

 spire, the whorls increasing in size with the growth 

 of the mollusc from the point to the base, where the aperture 

 is situated. The whorls turn round a column formed in the 

 centre, so that the section of a shell of this structure is ex- 

 actly like a turret stair with a solid newel, and the animal 

 which formed and inhabited it would be shaped like a cork- 

 screw. Here is a diagram, shewing a section of such a 

 shell to a large scale. Here we may follow clearly the 

 formation of the shell, if we suppose it to be still further 

 developed by rapid additions to the outer, and slow additions 

 to the inner lip. It is obvious that the termination of the 

 column forming the inner lip will be advanced, and that 

 the outer lip will be carried round it, and another whorl 

 will be formed in this shell, the form of which is the 

 type of a great many genera, all of small size, the Terebra 

 being the largest. The growth of ;the molluscs has been 

 gradual, and every whorl which has accompanied it bears 

 the same proportion to the whorl above it, but this is not 

 always the case, in the Triton for example, the same 

 form is seen to a certain extent from the apex of the shell, 

 after which the body of the mollusc takes a sudden develop- 

 ment, accompanied by the last whorl, called thence the 

 body whorl, which is out of all proportion to the whorls 

 above it. In some of the Volutes again, as in the Voluta 

 Neptuni, the spire, although distinctly developed, forms 



