152 POPULAR BRITISH CONCHOLOGY. 



capacity for enduring various conditions of existence, and 

 flourishing at various depths of the water, or on the shore 

 in sandy and muddy places. 



When walking, they wave their tentacles alternately from 

 side to side ; when at rest, the foot is much contracted, and 

 the horns withdrawn into the shell. Their motion on the 

 ground is very rapid; and they are also possessed of a 

 means of floating, foot uppermost, on the surface of the 

 water : to efi'ect this, they have the power of confining bub- 

 bles of air sufficient to sustain them. 



Some of the species afford nourishment to water-birds, 

 who pick them up greedily from the muddy and sandy 

 bottoms on which they crawl. 



Y. Jeffreysia. — Although there is nothing to distinguish 

 the shells of this genus from those of Rissoa, the animal is 

 very different, for it has four tentacles, two long and two 

 short ; and the eyes are placed far back behind the tentacles, 

 so as to come within the shell, which most likely, for that 

 very reason, is in both the species very transparent. The 

 shells of 



/. diaphana, which is long; and 



/. opalina, which is shorter and globose, are very simply 

 formed, glassy, and hght, admirably adapted to the 



