148 The Under-Water World 



their shells. Single fossil shells nearly 

 two feet across are commonly used as 

 doorsteps and garden decorations, and 

 it is probable that at one time the seas 

 flowing where Dorset stands to-day 

 swarmed with countless myriads of nau- 

 tilus-like animals. The living Nautilus 

 and Argonaut are both inhabitants of 

 tropical seas. In the former both sexes 

 are encumbered with massive shells, but 

 in the latter the shell is a female pos- 

 session only and is used as a receptacle 

 for the eggs. Two of the animal's ten 

 arms are tipped with large spherical 

 lobes that serve the purpose of the 

 ordinary molluscan mantle, and by 

 collecting carbonate of lime from the 

 water build up this delicate mid-ocean 

 cradle. The poets of old insisted that 

 the argonaut spread its arms to catch the 

 wind a charming conceit, but inaccurate 

 in the light of recent research. 



The cuttle-fish whose " bones " are 



