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The only source I have ever seen that spells his name "Merrill" is the single source in the main article (History of the Territory of Wisconsin). Any comments are welcome, but I am going to correct this by creating an article "Henry Merrell" and redirecting "Henry Merrill" (not right away, but eventually).
Also note that I am sympathetic to what happened to the Ho-Chunk, but merely throwing out words like "fraud" in what was a somewhat more complex situation is not helping to clarify things. So when I do re-edit the article I am going to pull that out unless this can be explained more by the editor who inserted it. But also, the place to put that information (the background and results of the various treaties with the Ho-Chunk) is in the article Ho-Chunk Nation of Wisconsin which itself needs a lot of work.