10 CALIFORNIA SEA-SIIELLS. 



ral lines are those which follow the course of the 

 tube ; lines of growth are cross lines, and mark 

 the successive positions of the outer lip. A shell 

 is in proper position to describe when it is placed 

 with the apex uppermost and the aperture fac- 

 ing the student. Dextral shells have tho apert- 

 ure on the right side ; they constitute the great 

 bulk of the univalves ; a few species have the 

 aperture on the left, and are called sinistral, and 

 occasionally a left-handed specimen of other spe- 

 cies may be found. 



In the bivalve shells, the right and left pieces, 

 or valves, are united bp a hinge, which is formed 

 of variously shaped, interlocking teeth. The 

 office of the hinge is to hold the two valves firm- 

 ly in place, and prevent their slipping in any 

 direction. These hinge teeth are to be carefully 

 studied, when determining the name of a species. 

 The central teeth, found just below the umbo, or 

 apex of each valve, are called cardinal teeth ; 

 those at the side, which are usually long and 

 narrow, are lateral teeth. Near the hinge teeth 

 is the ligament, or spring, composed of a dark, 

 elastic substance, like rubber. Sometimes it is 

 internal, and placed in a groove or pit. When 

 the shell is closed, the ligament is compressed, 

 and tends to throw the valves apart as soon as 

 the muscles cease to act. In other shells the 

 ligament is external, (see Fig. 2, Plate I), and is 

 stretched by the closing of the two valves. In 

 either case it tends to make the valves gape. 

 The opposing force, which tends to close them, 

 resides in the adductor muscles, which reach 

 from one valve to the other. A few molluske, 



