69 
the aperture is much narrower, and the upper portion of the 
aperture is distinctly arched. Figure 14d, Plate VII, shows one 
specimen with an aperture wider than usual, but figures 14e, f, 
show the characteristic differences. Many of the specimens of 
obrussa decampi have additional secretions inside the aperture 
by which the latter is still further constricted. 
Lymnaea galbana Say 
Plate VII, figures 15a-15f 
Shell small, robust, solid, subovate; surface shining, lines 
of growth distinct though not conspicuous. Whorls five, con- 
vex, roundly shouldered, increasing rapidly in diameter, the 
body whorl being much longer than the others; sutures deeply 
impressed; spire short, broadly conical, aperture ovate, rounded 
above and below; outer lip simple, acute; inner lip reflected 
over the parietal wall as a thin callus which emargins the umbili- 
cus. Umbilicus narrowly open, but occasionally shell is imper- 
forate. 
H. 9-5 W. 5-5 A.H. 5-5 A.W. 3-2 
Locality. Marl bed, McKay lake. 
This is a very characteristic fossil of post-glacial time and 
like the preceding species seems to have been well adapted for 
the cold waters of that period. It is a more robust shell than 
the preceding species. Its whorls are not as flatly shouldered 
as those of L. obrussa decampi and its aperture is not so narrow 
and is more equally rounded above and below. 
Planorbis antrosus Conrad 
Plate VII, figures 16a-16i 
Shell sinistral, discoidal, angulated. Surface covered by 
fine, numerous, oblique lines of growth and occasionally by 
irregular revolving striae. Apex minute, but can be seen at the 
bottom of the deeply depressed spire; whorls thin, rapidly 
increasing in height and width, discoidal, sharply carinated 
above and below; spire exhibits all the volutions in form of a 
depressed cone; aperture regularly lunate ovate, sharp above 
and more broadly rounded below. Top of aperture rises far 
above the preceding whorl ; peristome thin, expanding slightly, 
