108 WEST AMERICAN SHELLS 
a ledge of rock far into the ocean, which is com- 
monly known as Duxbury Reef. One morning 
when the tide was low I went far out on this reef, 
which forms a fine place for the home of many 
creatures. Turning back the masses of olive- 
green seaweed, I found a considerable number of 
mollusks with shells like the one shown in Fig- 
ure 94. It is not a very common species for Cali- 
fornia, but is more abundant in the north. 
The shell of a full-grown 
specimen is an inch and a 
half long. It is spindle- 
shaped, that is, it is largest 
in the middle and tapers 
towards each end. Various 
parts of the shell have re- 
ceived names, and as this is 
the first species which we are 
to consider which belongs to 
the great class of the Gaster- 
opods, we will now consider 
these names. The Gasteropod shell is really a long 
tube, coiled in a spiral form. This may not seem 
so evident in this species as in certain others, but 
it is the plan of the shell, nevertheless, and will 
help to explain many troublesome questions. 'The 
opening to this tube is called the aperture, and is 
marked ap in the figure. As you face the shell 
you notice that this aperture is on the side next to 
your right hand. This is true of most shells, which 
are therefore called dextral, while a few, which 
turn the other way, as in Fig. 142, are called sinis- 
tral shells. 
