10 CALIFORNIA SEA -SHELLS. 
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 sintstral , 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 th <$ 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 bj T 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 mollusks, 
