UNIVALVE SHELLS. 



477 



Fi- 97. 



Turret ed. When the whorls are many, and form a spire 

 longer than three diameters of the body-whorl. Ex. Turri- 

 tella. 



Fusiform. When the shell is thickest in the middle or 

 body-whorl, and tapers towards both the apex and base. 

 Ex. Fusus. 



Earsliaped. When the spire is niinnte and the body- 

 whorl very large proportionably, and widely open. Ex. 

 Haliotis. 



Involute. When the whorls form a retroverted spire, 

 which is bent in upon the body. Ex. Argonauta. 



Convolute. When the whorls are w'rapped round the 

 axis so as to embrace each other. Ex. Conus, Bulla. The 

 aperture of a convolute shell is always parallel to its 

 length. 



The Cypraea (Fig. 97) is a 

 convolute shell in our defini- 

 tion, but Linnaeus describes it 

 as being involute, the margins 

 of the aperture being rolled in 

 when the shell is fully grovvn 

 and perfected. But the figure 

 of the young and mature Cy- 

 prEea is very different. The 

 perfect shell, by an addition of 

 calcareous matter to the edges 

 of its lij)s, as goes on in the 

 formation of every shell, would 

 soon have the aperture entirely 

 closed, as you will perceive on 

 examining any species of that 

 genus. To get rid of this diffi- 

 culty, Bruguiere and others 

 have imagined that the animal 

 threw off" the shell when it had become too small for his 

 necessities, and then formed another more capacious, and 

 better fitted for his ease. This theory labours under insur- 

 mountable difficulty ; nor does it seem required by the 

 circumstances of the case. The Cyprgea?, in their immature 

 state, have a very different form from that they have 

 when full grown. When young, they are very thin and 

 brittle, with an evident spire, and a wide aperture, the 

 margins of which are not toothed and inflected, but 

 plain and efflise (Fig. 98). They are then, in fact, con- 

 volute shells of the ordinary character, and are obviously 

 enlarged, like all others, by the addition of matter to the 



