AIR-BREATHERS 153 
angular keels. Umbilicus large, shell thin and 
horny, diameter one-fourth of an inch. From Cal- 
ifornia lakes. 
Figure 134 gives us a good view 
of a mollusk that is very widely dis- 
tributed, many of whose varieties 
have received special names. They 
are, however, considered as all be- 
longing to the one species, which 
is named Planorbis trivolvis, Say, the Three- 
whorled Planorbis, (Helisoma trivolvis). In spite 
of its name the flat coil may consist of about four 
whorls. The color is hight brown, and the aper- 
ture is irregular. The cut represents a rather 
large specimen. 
Planorbis parvus, Say, the Little Planorbis, 
(Gyraulus parvus), has a very flat shell, con- 
sisting of a coil of fine, horn-like shell-tube. The 
whorls are about four in number, and the whole 
coil is from an eighth to a fifth of an inch in dia- 
meter. It occurs all through the Hast, also in the 
waters of the Pacific States. 
Planorbis vermicularis, Gld., the Worm-like 
Planorbis, (Gyraulus v). The tube of this little 
flat shell rapidly increases in diameter. Orbicular 
above, flattened beneath, same size as the last spe- 
cies; found in Washington, Oregon and Califor 
nia. 
