The Clam. 47 



be protruded from any part of the lower side through the 

 large opening in the mantle. 



Id many parts of the coantry the fresh-water clam vill be the one 

 most easily obtained. The manner of keepiug it in the sahoolroom 

 has already been described. 



A horny brown skin sometimes covers the whole shell 

 of the fresh-water clam, but is usually worn off near the 

 beak by the action of acids in the water. 



Fig. 2 shows the position of one of these olama in orawling, the 

 line at s representing the bottom of a lake or river, above which is 

 water. 



The beak is far forward, and the long hinge ligament, 

 being on the outside, acts as in the quahog, like a spring 

 to open the shell. On the inside of the shell are several 



Fig. 8 



hinge-teeth, if the clam is a Uuio, generally two wedge- 

 shaped teeth on the left valve and one on the right, while 

 back of these the right valve bears one long lateral tooth 

 and the left bears two. 



In Fig. 8 two of the wedge-shaped, or cardinal teeth are shown 

 on a right valve at c, and the lateral tooth at /. 



The pearly lining of this shell is the same as that which 

 forms the mother-of-pearl of the pearl oyster. "When this 

 deposit is increased by a particle of sand or some small 

 object that has worked its way in between the mantle and 

 the shell, an imperfect pearl is formed. Since the clam 



