THE FRESHWATER MOLLUSC HELISOMA CORPULENTUM AND 
ITS RELATIVES IN CANADA 
By F. C. Baker 1 
CONTENTS 
Page 
Introduction 1 
Revision of the groups Helisoma corpulentum and Helisoma trivolvis in Canada 4 
Group of Helisoma corpulentum 5 
Group of Helisoma trivolvis , 11 
Identification of corpulentum references in the literature 23 
References not included in the above bibliography 26 
Illustrations 
Plates I-V, Illustrations of species 28-37 
Figure 1. Shell of Helisoma showing methods of measurement 4 
2. Sculpture of Helisoma corpulentum 9 
3. Sculpture of Helisoma corpulentum multicostatum 9 
INTRODUCTION 
In 1824 the great American naturalist, Thomas Say, described a mem- 
ber of the old genus Planorbis as Planorbis corpulentus, giving as localities 
Winnipeg (Winnipeck) river, Winnipeg (Winnipeck) lake, Lake of the 
Woods, and Rainy lake, all in Manitoba or western Ontario. Owing 
probably to the rarity of this species in collections, although it has been 
found by later exploration to be an abundant species in Canada, it has 
been generally misunderstood by most subsequent students of the Mollusca 
down to the present time. A few isolated instances occur of quotations 
relating to the true corpulentum. In 1900 (Nautilus, XIII, page 133) Dr. 
Bryant Walker, a critical and careful student of freshwater molluscs, rescued 
the species from oblivion, correctly calling attention to Say's very exact 
description of the species (and to his figure) which, when read with a 
specimen of the true corpulentum in hand, is excellent. 
The researches of Dr. Alvin R. Cahn in the waters of central and 
western Ontario (See Ann. Rept., Nat. Mus. of Canada, 1929, page 41) 
resulted in securing the largest series of specimens of this species that has 
ever been obtained, far in excess of material in all previously known 
collections combined. A study of this material has shown that the species 
is a composite one, embracing several varieties and at least one additional 
species, which have been previously referred to corpulentum. With this 
wealth of material it has been possible to clear up all doubts concerning 
the status of Say’s species, to ascertain its probable geographic distribution, 
and to distinguish its variations and related species. More than five 
hundred specimens of corpulentum and related species and varieties have 
been examined in the course of this investigation. 
i Contribution from the Museum of Natural History, University of Illinois, No. 75. 
9503—1* 
