4 
REVISION OF THE GROUPS HELISOMA CORPULENTUM AND 
HELISOMA TRIVOLVIS IN CANADA 
The large snails of the genus Helisoma divide into two well-defined 
groups, corpulentum and trivolvis, around which several species and races 
arrange themselves. In the corpulentum group the upper and lower mar- 
gins of the body whorl have the carina bordering the outer edge, presenting 
a flattened body whorl separated from the dorsal and ventral surface by a 
sharp ridge. In the trivolvis group the carina is placed in the centre of 
the basal and spire whorls and the body whorl is usually quite rounded. 
The various species and races differ not only in shell characteristics but 
also in the size and numerical formula of the teeth on the radula. The 
genitalia are fairly uniform for both groups (See Proc. Zool. Soe., London, 
1931, page 575, where the genitalia are figured). It is to be noted that the 
generic name Helisoma is a Greek neuter noun and that all specific names 
as adjectives must also be neuter, thus Helisoma corpulenta as previously 
understood becomes Helisoma corpulentum (See H. B. Baker, Nautilus, 
XLIII, page 319, 1930). 
S 2 
Figure 1. Shell of Helisoma showing methods of measurement. 1-1, greater diameter 
(from behind expanded lip); 2-2, height; 3, height of aperture; 4, diameter of aper- 
ture; 5-5, height of aperture with expanded lip. 
A word concerning the method of making measurements of the shells 
considered in this paper may not be out of place, since it may differ from 
the method used by some conchologists. The annexed figure in the text 
sufficiently explains the method used without further description (Figure 1). 
The figures on the accompanying plates are from typical specimens of each 
species or race, the types being used in many cases for this purpose. The 
plate of immature shells is of importance in showing that certain differ- 
ences are shown in the young shell which may more or less disappear in 
some adult specimens from certain localities. The catalogue numbers and 
locations of all specimens used for illustrative purposes are given to facili- 
tate future consultation. It is to be noted that the shells of Helisoma are 
sinistral and not dextral and in this respect Say was correct in his early 
descriptions. 
