186 SHELLS AND MOLLUSCOUS ANIMALS. 



of their own bodies, and some by the help of a foot 

 shoyel out the sands, and bury themselves in a hole 

 of their own making. The hinge which unites the 

 valves of the shells has a saw-like edge, or little 

 teeth, which fit exactly into corresponding cavities. 

 The beautifully constructed elastic hinge binds it 

 together, and is designed for opening the shell; 

 while the animal is endowed with one or more 

 muscles, by which it can close it, and keep it firmly 

 shut. The valves of some bivalve shells are forme'd 

 exactly alike, while others are dissimilar. Some- 

 times one is smooth, the other rough ; frequently, 

 one is flat and the other convex, and often one 

 shell is shorter than its companion. 



Among the most common of our bivalves are 

 those deeply violet-tinted shells of the common 

 mussel (MytHus edulis), which are found in great 



abundance on rocky shores, fastened by no slight 

 hold to the rocks, and, however wildly the waters 

 may dash over them, are rarely loosened from their 



