116 GEOLOGY OF LA SALLE COUNTY. 



Shell is larg-er than the other, and this is called 

 the anterior or forward end, the other the posterior 

 The back is g-enerally broad and somewhat swollen 

 or inflated toward the anterior end, forming- a raised 

 space on either side, w^hich in some cases projects 

 back and outward beyond the line of the hing-e so as to 

 appear like horns. These are called umbones and are 

 sometimes curved forward. Around these the shell 

 seems to be built up in nearly concentric layers. 



The shell is closed by two powerful short muscles, 

 one near each end, by contracting- which the valves are 

 drawn tog-ether, and when they are relaxed the valves 

 spring- apart. The animal breathes by means of g-ills 

 and extracts its food from water which it draws in at 

 the anterior end throug-h two tubes called siphons and 

 expels at the opposite extremity. They move along- 

 the bottom of the stream or pond by expelling- water 

 with g-reat force from their siphons. They can also 

 extend the thick, fleshy foot, and attaching- it to a 

 stick or stone pull themselves up to it, and thus move 

 slowly along- the bottom, wherever there is anything- 

 of which they can g^et hold. The flesh is hard, tough, 

 rather tasteless, and the longer they are cooked the 

 harder and toug'her they become. Some people, how- 

 ever, eat them. Raccoons and muskrats are fond 

 of them, and chickens greedily devour them when 

 divested of their shells. 



These creatures, dull, senseless, helpless, as they 

 appear to be to us, are, nevertheless, able to repair 

 their shells when broken, and to coat the insides of 

 them with a smooth layer of pearly nacre, if by any 

 means the}^ become roug-li. They prefer muddy bot- 

 toms, but are frequently found on sand, where the 

 water is charged with much decayed vegetable matter. 



