FRESH-WATER MUSSELS. 



29 



early stages in the growth of the shell are usually destroyed. 

 In very yoimg shells, however, the early stages can be plainly 



seen. 



Back or dorsal region. 



VeDtral region. 



Fig. 35. — A ITeesh- Water Mussel. — 2, Ligament ; «. Umbone ; /, Foot ; as., Excurrent 

 Orifice; in., Incurrent Orifice. 



28. The ligament is always behind the beak, or umbone, 

 in fresh-water mussels, and in nearly all hivalve shells (so 

 called, because they have two valves or pieces, while the 

 snail-shells are sometimes called univalve shells, because they 

 have but one valve or piece). 



Let the pupil now hold a perfect mussel-shell in his hand 

 (that is, a mussel in which both valves are together, and 

 united across the back), with the ligament uppermost, and the 

 umbone away from him, or beyond the ligament, and the 

 valve on Jiis left hand is the one which covers the left side 

 of the animal, while the valve on his right hand covers the 



