2 
It was also believed that the positive identification of all previous 
records of corpulentum would be of value and, accordingly, this was 
attempted. To carry out this plan successfully necessitated the examina- 
tion of all the available original specimens upon which the old records 
were based. This has been made possible by the hearty co-operation of 
the large museums in which this material is now deposited, principally the 
National Museum of Canada, the United States National Museum, the 
American Museum of Natural History, the Academy of Natural Sciences 
of Philadelphia, and the Zoology Museum of the University of Michigan. 
The author has also drawn upon Dr. Walker’s article in the Nautilus, 
The examination of this material, so generously placed at the dis- 
posal of the author, plainly shows that this fine northern species has been 
totally misunderstood by nearly all previous students. Upward of eleven 
different species and races have been confused with this species, trivolvis, 
binneyi, macrostomum, tenuis, intercalare, pilsbryi, ammon, occidentale, 
vermilionense, multicostatum, and injracarinatum, the last three new forms 
recently diagnosed. Binneyi and trivolvis have been the species most 
usually mistaken for corpulentum, and the former bears considerable 
resemblance to the figures of Say. The reference of corpulentum to this 
western form in Binney’s work (1865), based on Gould’s previous disposi- 
tion of the species (1852), perpetuated this error, in spite of Tryon’s later 
correction (1867) in which he bestows the name binneyi on the western 
form, but states that corpulentum is a synonym of trivolvis. With the 
single exception of Grant (1887), the species was totally misunderstood 
previous to the publication of Walker’s paper in 1900. The previous 
references to this species and their present relationship will be found in 
the literature identifications in this paper. 
In the discussion of the species included in the corpulentum- trivolvis 
complex an effort has been made to state the salient diagnostic features 
of each form, the type localities and present location of the type material, 
and a detailed list of the material contained in the museums of the institu- 
tions from which specimens were loaned, with their collection numbers, 
this for the convenience of later students who might wish to check up any 
of the citations. The material in the Natural History Museum of the 
University of Illinois has also been used in the investigation. The speci- 
mens figured on the plates are mostly in this museum. 
In a previous paper (Ecology, XI, page 469) the writer called attention 
to the significant geographic distribution of certain species of freshwater 
molluscs in relation to the moraines of the last glacial invasion, the Wis- 
consin. In a summary (page 479) the following statement is made: 
“ Previous to the Glacial Period the country had been reduced to base-level and 
probably few lakes existed, the physiography being one of rivers with dendritic form 
of drainage, like the driftless area in Wisconsin today. After the last invasion, the 
Wisconsin, the country was greatly changed; in place of rivers there were lakes, 
swamps, and sluggish rivers. The fauna reacted to this change to such an extent 
that where previously there had been but one or two varieties in a species, as many 
as ten developed which were peculiar to the newly glaciated country. Many entirely 
new species were evolved which have not occurred in any glacial deposits yet examined. 
The change affected some species more than others, but all have been affected to a 
noteworthy degree.” 
