296 
PROCEEDINGS OF THE NATIONAL MUSEUM. 
VOL. XXIV. 
THE PLANORBES. 
Of the numerous species of Planorbis inhabiting North America, 
the larger forms occur west of the Kockj Mountains and north of lat- 
FiG. 7. — Planorbis trivolvis 
Say (original). 
Fig. 8. — Planorbis trivolvis 
Say, distorted (original). 
itude 30° N. Thus we find P. {IIclisonKi) frlvolois of conspicuous 
size, as is shown hj the accompanying figures. These represent 
examples from the cool Avaters of mountain lakes 
where the elevation is about 5,000 feet. 
Another characteristic form peculiar to the West 
coast is 1\ {Ilelisoma) aminon. The metropolis of 
this species is apparentl}^ the Colorado Desert where, 
as previously stated, it is found in the greatest 
abundance. It is distinguished not alone b}' its 
rugged and rather irregular growth, l)ut b}" the mass 
of the soft parts as seen in living examples, which 
probably exceeds that of any other species. In the 
case of this form we lind it scattered over a large 
area at 200 feet below the level of the sea, where 
the waters of contiguous springs have a temperature 
of 100° F. 
These forms exhibit a swollen ventricose irregu- 
larity and patulous expansion of the aperture atti'ibu- 
table to visceral hypertrophy, quite as likely as to 
salinit}^ and the distortion of the example of /''. tri- 
volvis, above figured, is presumably due to the same 
cause, as the specimen was collected in the sweet 
waters of a mountain lake. 
Fig. 9. — Planorbis 
AMMON (Gould). 
