17 
with either hornii or plexatum. This species appears to be specifically 
different from plexatum or hornii, the three species varying around three 
easily recognized types of Helisoma. 
The species here referred to subcrenatum is widely distributed in the 
United States and Canada. In the former territory it is known from 
Washington, Idaho, and Utah. In Canada it has been identified from the 
following places: 
New Osgoode, Saskatchewan (Royal Ontario Museum of Zoology, No. 6830). 
Mackenzie district: Fawn lake, mouth of Hay river; Mackenzie river, 30 miles 
above Providence (Whittaker, Nautilus, vol. 38, p. 11); Little lake, west end 
Great Slave lake (Nat. Mus., Canada, 3148, Plate III, figures 15-16). 
Aiberta: Wainwright park, several lakes; British Columbia: Cranbrook and Fernie 
(U.S.N.M., coll, by 0. Bryant). 
Of the trivolvis group there are thus seen to be three distinct species, 
two of which have been referred to trivolvis as synonyms, and the third 
totally misunderstood. The distribution of these species will be found to 
cover much of western, central, and northern Canada when more material 
has been studied. The molluscan fauna of the great stretch of territory 
in northwestern Canada is very little known. 
In studying large quantities of Helisoma trivolvis from various parts 
of the United States a size variation coincident with geographic range may 
be noted. The trivolvis of Mississippi valley, now referred to the race 
pseudotrivolvis, are all relatively small, rarely exceeding 20 mm. in 
diameter. Typical trivolvis is smaller in the southern part of its range, 
also. As the range passes northward into Wisconsin, Michigan, and Min- 
nesota, the size increases and the aperture becomes more expanded. 
Specimens of this nature, from a pool near Presqu’ile, lake Huron, Michi- 
gan, are figured on Plate III, figures 17, 20. These so-called transition 
specimens lead to the race named macrostomus by Whiteaves. 
Helisoma trivolvis macrostomum (Whiteaves) 
Plate III, figures 18, 19, 21, 22 
Planorbis macrostomus Whiteaves, Can. Nat. and Geol., VIII, p. 113, 
Fig. 12 (1863); Binney, L. and F.-W. Shells N.A., II, p. 119, fig. 
199 (1865) ; Dali, Alaska Moll., p. 89, fig. 69 (1905) ; Tryon, Con. 
Hald. Mon., p. 202, PI. 6, fig. 16 (1872). 
Helisoma trivolvis Baker (part), Fresh Water Moll. Wis., I, p. 330, 
PI. xx, fig. 13 (1928). 
Shell much larger than typical trivolvis, the whorls swollen, rounded 
below and subcarinate above; spire sunken below the level of the last two 
w T horls, the last w r horl in many cases overlapping the previous whorl; base 
excavated and umbilicus deep, exhibiting 3 to 3J whorls; whorls 4^ to 5, 
rapidly increasing in diameter; sculpture of distinct, widely spaced riblets 
with growth lines between and crossed by spiral striae; aperture V-shaped 
above, widely expanded and rounded below; outer lip strongly reflected 
