PTJEPLES. 
19 
to aperture run thirteen spiral grooves, giving it 
the appearance of 0. dims. It can easily he dis- 
tinguished by its shorter spire, smooth Jip, and 
deep suture. It is of light color, sometimes sti'eak 
with reddish brown. 
Purpura crispwta , Ohem., Rough Purple, Fig. 
6, PI. II. is found in San Francisco Bay, but is 
more common in more northern regions. It is 
often two or three inches long, strong and heavy' 
The spire consists of four strongly grooved 
whorls. The body whorl is sometimes smooth, 
but often very rough and foliated. It is said to 
change the habit of its growth, when moved into 
new localities. Its color is white or light brown. 
In old shells, the umbilicus is conspicuous. Like 
the other purples it has a horny, elongated oper- 
culum. Fig. 1, PL I, represents the same species. 
Next comes a genus of mollusks, almost pecu- 
liar to the west coast of America. A good rep- 
resentative is shown in Fig. J, PI. Ill, Mono- 
ceros lapilloides , Conr., Pebbly Horn Shell. It 
is so named on account of a little horn, near the 
base of the outer lip. It is a pretty little shell, 
found on the rocks between tides, having a spire 
of four whorls, a rather small white month, set 
with seven teeth. The outside is marked with 
spiral grooves, and the lines of growth break up 
the brown color into little blocks. The shells 
are very thick and strong, well fitted to resist 
the force of the waves. Thin and delicate shells 
are generally found in deep water, where there 
is little danger of being dashed against the rocks. 
Shore shells are usually strong and solid. When- 
ever you examine a shell, notice these points 
