154 WEST AMERICAN SHELES 
Planorbis ammon, Gld., the 
Ram’s-horn Planorbis, (Helis- 
oma ammon), is a species found 
in the San Joaquin river and 
other bodies of fresh water in the 
central part of California. Fig- 
ures 135 and 136 shows that the 
shell is large, robust, with a cup- 
shaped depression both above and 
below. As the animal grows it 
winds its shell 
round) and 
round in the 
same plane, in- AS Qs 
stead of making ap 
ae a spiral as in the Tie. 135 
ease of most mollusks. Hence the 
shell is called Planorbis, or Flat-coil. The aper- 
ture of the shell is large and ear-shaped; the color 
is a rich yellowish brown, but is white within the 
aperture. The lines of growth are very distinct, 
and mark the shell in a pleasing manner. 
Planorbis bicarinatus, Say, the 'Two-keeled 
Planorbis, (Helisoma bicarinatus), resembles the 
last species, but is much smaller. The whorls have 
a sharp angle, or keel, both above and below the 
suture. Specimens were collected at Portland, 
Oregon. 
Acroluxus nuttallii, Hald., Nuttall’s 
River-limpet. The shell of this little crea- 
ture is shown in Figure 137. It is gen- Fis. 137 
