2l8 



AN INTRODUCTION TO ZOOLOGY. 



with two equal adductor muscles, include the Sinupal- 

 lialia and also the cockles and the fresh-water tnussels 



Fig. 71. — Shell of the Fresh-water Mussel, Anodtrnta cygma. 



Anodonta and TJnio, so abundant in our streams. The 



Fig. 72. —Shell of the River Mussel, Umo liioralis. 



latter is the source of small seed-pearls, which in the 



