An entry in his journal by Private Joseph Whitehouse, a member of the Lewis and Clark expedition, on his travels through the Rickoree nation. The entry is dated Saturday, September 29, 1804:
Saturday 29th Sept. 1804. we Set off eairly. proceeded on passed bluffs on S. S. Saw Several Indians on Shore 1 or 2 of the brave men as they called themselves, wanted Some tobacco. the Officers gave them 2 carrits of tobacco but told them that we Should not Stop untill we Got to the RickRee I. Nations. passed an old village on S. S. where the RickaRees had lived 5 years ago, had raised corn beans [peas and Simblins?]  Camped on a Sand beach on the S. Side.—
Saturday Septemr. 29th We set off early this morning, having fine clear Weather, and passed by several Bluffs lying on the South side of the River, we saw several Indians on the shore as we passed along, One or two of them, (brave Men as they called themselves,) told the Officers that they wanted some Tobacco, The officers gave them two Carrots of Tobacco, and told them, that we should not stop ’till we got to the Rickoree nation, We proceeded on, and passed an old Indian Village, lying on the South side of the River; where the Rickorees had lived five Years before; and we were inform’d by one of the frenchmen, that was with us, that they had raised Corn, Beans, pease & Simblins at that place, We proceeded on, and encamped in the Evening, on a Sand Beach lying on the South side of the River.—
“Simblins” is followed by a question mark, maybe indicating that the transcriber had difficulty in reading the text, and the entry is footnoted, although the footnote’s author isn’t identified:
6. The peas could be Indian potato, or hog peanut, Amphicarpa bracteata (L.) Fern. Simlins are summer squashes.
I found a second reference in the online version of a book by Craig Thompson Friend entitled The Buzzel about Kentuck: Settling the Promised Land (page 70):
At fisher’s Garrison in present Boyle County, John Hinton was killed in 1781 while retrieving “simblins” (a type of squash) from a storage pit outside the stockade.
Also, across the pond in Lancashire, a “poet and author in the Lancashire dialect” named Edwin Waugh (1817–1890) made this observation about his district:
On “Simblin-Sunday,” the two towns of Bury and Heywood swarm with visitors from the surrounding country, and “Simblins” of extraordinary size and value are exhibited in the shop-windows.
And finally there is this quote, in some ways the most interesting, from the Bolton Evening News of March 20, 1873 (Bolton being another Lancashire town):
NEXT Sunday being ‘Simnel’ or Mid-Lent Sunday, the confectioners of the town have made the customary extensive preparations for the extra-ordinary demand for these ‘cracknels’ or what at better known in this neighbourhood as ‘simblins’. Perhaps the largest specimen of these simnels is to be seen in the window of Mr Henderson, Town Hall Square, which weighs upward of two cwts, and is tastefully decorated. The other leading confectioners in this town display very beautiful and rich samples of this kind of confectionery, the simnels varying both in size and decoration, and are such as will satisfy the wants of all classes of purchasers.
Based on all this, it looks like Twain meant some kind of summer squash and put “simblins” in quotes because it was recognized to be a regionalism.
But now we arrive at the ultimate authority, which I originally consulted but managed to read right over the entry: I mean, of course, the venerable Oxford English Dictionary. My friend and fellow writer Joe Nigg noted the OED’s “Simblin” entry calls it “a variant of Simlin, and defines “Simlin” as “A species of squash having a scalloped edge,” i.e. Cucurbita melopepo. However, the earliest citation in the entry specifies the Latin name as Cucurbita verrucosa, which is a long-neck squash
So … which of these are “simblins”?