3 
The study of the Helisomas of the corpulentum- trivolvis groups bears 
out this assumption in a marked manner. None of the coijmlentum group — 
corpulentum, vermilionense , multicostatum, whiteavesi — is known south of 
northern Minnesota, and they are mostly confined to western and central 
Ontario. Macrostonium, pilsbryi, and injracarinatum of the trivolvis group 
are distributed far to the north of the Wisconsin ice border and none has 
been found in geological deposits south of this area. We must conclude, 
therefore, that following the last glacial advance there was an acceleration 
of evolutionary factors resulting in the appearance of the large species and 
races of Helisomas so abundantly distributed throughout Ontario and other 
parts of Canada. The parent stock of these forms would appear to have 
been the widely distributed trivolvis of the central and eastern part of 
the United States. That the drastic change in the environment played a 
large part in directing this evolution is the belief of the author. 
ACKNOWLEDGMENTS 
The success of the present paper is largely due to the co-operation of 
the following persons who generously loaned much,, valuable material in 
their institutions for study and comparison, in addition to supplying con- 
siderable material from their own collections: 
Dr. Paul Bartsch, U.S. National Museum, Washington, D.C. 
Mr. Calvin Goodrich, Museum of Zoology, University of Michigan, 
Ann Arbor, Mich. 
Dr. Wm. J. Clench, Museum of Comparative Zoology, Cambridge, 
Mass. 
Prof. J. R. Dymond, Director, Royal Ontario Museum of Zoology, 
Toronto, Canada. 
Dr. F. Kermode, Director, Provincial Museum, Victoria, B.C. 
Mr. J. Henderson, University of Colorado, Boulder, Colo. 
Mr. A. LaRocque, Geological Survey, Canada, Ottawa, Canada. 
Mr. Wm. B. Marshall, U.S. National Museum, Washington, D.C. 
Dr. Roy W. Miner, American Museum of Natural History, New 
York, N.Y. 
Dr. H. A. Pilsbry, Academy of Natural Sciences, Philadelphia, Pa. 
Prof. D. S. Rawson, University of Saskatchewan, Saskatoon, Sask. 
To Dr. A. R. Cahn, of the Department of Zoology, University of 
Illinois, special acknowledgment is due for the splendid collections of 
Helisoma gathered during the years 1928, 1929, and 1931 in parts of 
central and western Ontario. Without this material this revision of the 
northern species of Helisoma could not have been undertaken. 
