214 MOLLUSKS: ACEPHALS. 



food. Their beautiful shells are known to almost 

 every one ; for they are much used in making card- 

 holders, pin-cushions, and other little articles both 

 useful and pleasant to see. The pecten swims rapidly 

 by opening and shutting its valves. 



Sea-Mussels, Figure 433, inhabit mud-banks which 

 are uncovered at low water. They multiply rapidly, 

 and grow to their full size in one year. By means of 

 a collection of horny threads, called a tyssus, they at- 

 tach themselves to rocks, or to the ground. 



Pearl-Oysters, or Aviculas, Figure 434, have shells 

 which make the beautiful material called "Mother-of- 

 Pearl," which is extensively used for ornamenting fine 

 cabinet-work, and for making knife-handles, paper-cut- 

 ters, buttons, and a great number of other useful and 

 beautiful articles. They also yield the Oriental pearls. 



UNIOS, OR RIVER-MUSSELS. 



These mollusks abound in brooks, rivers, ponds, and 

 fresh-water lakes. They are sometimes called Na'ides, 

 and there are very many kinds. It would take sev- 

 eral books larger than this one to describe all the 

 kinds found in the United States. A few of the forms 

 of Unios are shown in Figures 437-441. Sometimes 

 beautiful and valuable pearls are found in these mol- 

 lusks. One of the pearls in the Royal Crown of Eng- 

 land came from a river-mussel. 



RAZOR-SHELLS, CLAMS, &c. 



The Razor-Shells are very long and smooth. They 

 burrow in the sand, and are good for food. The Com- 

 mon Clam burrows in sand and mud, and is exten- 

 sively used for food, and for bait for cod. 



