40 RAMBLES IN SEARCH OF SHELLS. 



the deep furrows which they make in the soft mud. 

 They feed on decomposed animal and vegetable 

 substances ; and the size and solidity of the shell 

 depends on the abundance of the food and the 

 state of quiescence or motion, and of calcareous 

 matter in the water in which they happen to reside. 

 These shells make capital cream- skimmers, and in 

 French dairies are used for the purpose. They are 

 procured by means of a long-pointed stick, which is 

 thrust between the open shells when the animal is 

 feeding, and these, closing on the stick, allow it to 

 be drawn up out of the water. 



Although a great many varieties of the common 

 swan-mussel have been described as different 

 species, we believe it is pretty generally admitted 

 that a second species of the genus (A. anatina) 

 exists in this country. It has a smaller shell, 

 which is longer in proportion, and the hinge line 

 is raised instead of being straight, while the pos- 

 terior side is abruptly instead of gradually sloped 

 off. It is of similar habits, and frequents similar 

 situations to the last named. 



The curious-looking zebra mussel (Dreissena poly- 

 morpha), although not indigenous to this country, 



