72 
H. 2-25 W. 6-0 A.H. 1-5 A.W. 2-0 
Locality. Colton lake. 
This small planorbis is rather uncommon in the marl beds, 
good specimens being rare. It is much larger than P. altissimus , 
from which it also differs in the subcarinate periphery of its 
whorls and the deflection of the last whorl. It probably is more 
widely distributed than is indicated by the single locality given 
above. 
Physa gyrina Say 
Plate VIII, figures 19a-19i 
Shell sinistral, polished shiny, subovate, growth lines fine 
but distinct, revolving lines faint or absent; whorls four, expand- 
ing rapidly; spire moderately elevated, sutures fairly deeply 
impressed; aperture very large, elongate-ovate, occupying over 
two-thirds of the total length of the shell; peristome thin, 
thickened just inside the aperture by a pronounced callus, which 
is frequently of a reddish colour, terminations of peristome 
connected by a callus on the parietal wall; shell imperforate; 
columella straight. 
H. 11*5 W. 7*50 A.H. 8*75 A.W. 4*25 
Locality. Marl beds, McKay, Colton, and Mink lakes. 
This form, which is quite common in the marl beds, resem- 
bles P. heterostropha Say. Its sinistral shell, short spire, and very 
large body whorl and aperture render it quite distinct from any 
other gastropod in the area. At present it is found in ponds 
and streams adhering to sticks and stones. 
Amnicola limosa Say 
Plate VIII, figure 22a 
Shell small, bulbous, surface shining, lines of growth very 
fine, numerous; sutures well impressed; whorls four, gradually 
expanding; aperture rounded ovate; peristome sharp, barely 
attached to the parietal wall of the preceding whorl and occa- 
sionally thickened slightly ; umbilicus almost covered and shell 
frequently nearly imperforate. 
H. 3 W. 2-25 A.H. 1*5 A.W. 1*15 
Locality. Marl bed, Colton lake. 
