34 



a very insigniftcant portion of tlie sUolI, which is all body 

 whorl, and this is the case with a great number of shuUa 

 which assume a globose form, in which, however, the 

 initial spire is small and very little raised, as in 

 the species of Paludina (for instance), and the 

 most common snails. In such shells as Nerita 

 of which an elarged diagram is here given, the spiral 

 form becomes more obscure. But this is not so obvious, 

 because the axis of the shell is in the direction of the 

 shorter, and the mouth in the larger measurement, contrary 

 to the other shells we have been looking at. In the Cyprsea 

 the spiral form becomes esceediugly obscure ; no one would 

 take a Cyproea for a spiral shell, but it is so nevertheless . 

 Here is a young Cyprrea showing the spiral apex, a wide 

 aperture, and sharp lip, which is afterwards thickened as it 

 approaches the inner lip, while the spire is buried so as to 

 become scarcely discernible in the mass of the shell. 

 I will now call your attention to these seams on the 

 back of the Triton. The deposition of the shell by the 

 mantle is not continuous ; it would appear that after a cer- 

 tain time the power of the animal becomes exhausted, an 

 interval of rest follows, after which another portion of shell 

 is secreted, and then another rest, and so on. These seams 

 mark the intervals of rest, and the spaces between them are 

 the portions of the shell formed at each period of activity. 

 Some molluscs, whenever an interval of activity is com- 

 pleted, form a mouth to the shell exactly like the final aper- 

 ture ; and in many cases, where the mouth of the shell 

 is thickened and reflected, the seams are marked accord- 

 ingly, each having been in its turn the mouth of the 

 shell. In the Murex imperalis, for instance, the mouth is 

 thickened, denticulated, and knobby, and this same forma- 

 tion, the knobs included, is repeated in every seam on the 

 back of the shell. But the case of the successive forma- 

 tions of the complete mouth is most remarkably shown in 

 the Murex Inflatus : the luxuriant and beautiful foliations 

 of the aperature have been repeated at the termination of 



