From 1838 Albert Koch exhibited a skeleton across America and Europe he named the 'Missourium' which he'd assembled from several Mastodon specimens, bulked up with wood to make it appear even larger.
James Pedder, editor of The Farmer's Cabinet and American Herd Book, reinterpreted it as a giant tortoise and sent a letter and sketch to Petty Vaughan dated November 20, 1841 outlining his ideas:
"After minute investigation, and repeated opportunity for examination of the Skeleton of the Missourium, I have been led to conclude that the animal was a Monster of the Tortoise Tribe 32 feet long and correspondent width, with the power of with-drawing its head within its shell; the tusks then forming a mail of defense around its edge to ward off obstruction."
Sketch published in Simpson, G. G. 1936. Misconstructing a mastodon. Natural History, 37: 170-172.
https://digitallibrary.amnh.org/items/535025b6-b184-41a9-8fdb-86ca5d56b849