198 PLANORBIS. 
Subgenus HELISOMA, Swainson. 
{A. SPECIES INHABITING THE PaciFic STATES. 
12. PLANORBIS AMMON, Gould. 
Plate 6, figs. 3, 4, 5. 
Pl. Traskii, Lea. Plate 6, fig. 6. 
Shell large, discoidal, subconic, finely or 
coarsely striate; left side broadly and deeply 
concave, showing four obtusely carinated 
whorls; right side concave, showing two-and 
a-half rounded whorls; aperture ovate-trian- 
gular, expanded on each side when fully 
grown. 
Diam. 25, alt. 15 mill. 
California, Oregon, Washington Territory. 
The specimens at different stages of growth 
vary greatly in the development of the aper- 
ture; but all agree in the peculiar slope of the 
outer volution, giving them a conical or dome- 
shaped form when lying on the left side. 
: (45) 
