9 
The corpulentum of the Canadian lakes divides into two well-marked 
groups, one with coarse sculpture and one with fine sculpture, the riblets 
crowded and evenly spaced. It is probable that Say did not see the form 
with fine, crowded riblets or he would not have described the shell as 
rugged with coarse wrinkles, features so characteristic of the true corpu- 
lentum, but would have distinguished between the two forms. So far as 
known the milticostatum form has not been found in the waters visited 
by Say. The crowded striae of the surface and the flatter base will readily 
separate multicostatum from corpulentum. Occasional specimens occur 
that are true corpulentum with widely spaced riblets for three whorls where 
an injury has distorted the shell, the remaining growth being true multi- 
costatum with fine, close-set riblets. Text Figures 2 and 3 illustrate differ- 
ences in rib-sculpture. 
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Figure 3. Sculpture of Helisoma corpulentum multicostatum showing close spacing of 
riblets. Line indicates one millimetre. Camera Lucida dr-awiug. 
Multicostatum is at once distinguished from Helisoma pilsbryi, which 
it somewhat resembles in its fine sculpture, by the flattened base and 
■carinated whorls. From the race vermilionense it is separated by its finer 
sculpture and lesser axial height w'hich is about like that of corpulentum 
