^^P«eftSANTSHeLL 



THE passage from the uncoiled 

 Limpets to the strictly spiral 

 cones of the Tops is made 

 easy by way of the Slit- 

 limpets and Scissurella, as 

 shown in the previous chapter. A further advance 

 in that direction is seen in the tiny Dolphin-shells 

 (Cyclostrema), of which we have three species. They 

 are more orbicular than Scisswrella, and without the 

 slit margin of the almost circular mouth. The coil- 

 ing of the shell round an imaginary axis leaves a 

 deep cavity, known as the umbilicus, which we 

 shall meet again and again from this chapter in 

 our story, and shall find that it is often a valuable 

 clue in the discrimination of species. Another feature 

 encountered for the first time is the operculum, a 

 thin horny plate attached to the hinder part of the 

 mollusk in such manner that when it withdraws into 



