O^it-shells and Mussels 55 



water, its living range being between 3 and 145 

 fathoms of water. The shell, as shown in the figure, 

 is bluntly triangular in shape, coloured 

 outside with yellowish green marked with 

 darker growth-lines running parallel with 

 the lower margin, whilst many very fine 

 grooves radiate from the rather prominent 

 beak to the lower margin. The little creature 

 that produces this shell is cream coloured ; its mantle 

 lias plain margins, and its oval foot is toothed all round 

 the edge. The mouth and lips are small, but the 

 lips have long appendages which are protruded from 

 the shell at the same time as the foot, and used as 

 sense-organs in the search for food. The small brown 

 gills are simple and plume-like. 



The maker of the Nut-shell burrows among sand 

 and gravel, the '■' foot " being extended to its utmost 

 length and worked between the fragments ; each tip 

 of the tooth-like edges then catches hold of a particle 

 of gravel, the foot as a whole is shortened, and so the 

 shell is hauled along by the contraction of the foot. 

 At the same time the appendages of the lips feel 

 about for food. When the dredge has scraped up 

 one of these little mollusks from the bottom there is 

 no foot visible ; it has been withdrawn into the shell, 

 and the valves now fit closely together. 

 Only the insertion of a knife to sever the 

 muscles, or to break them by leverage, can 

 effect the opening of the shell. The 

 examination of an odd empty valve, 

 picked up on the shore, or of the accom- 

 panying figure, will help us to understand how this 

 firm closure is maintained. It is a left-hand valve. 



