There's an excellent chance it's a contemporary (meaning "from the 19th century era") counterfeit. It just does not look like it's made of silver. What it could be is German Silver, an alloy of copper/nickel/zinc that might corrode this way. Many of these counterfeit coins circulated in the 1840s-1860s, and I suspect a lot of them were chucked into a local river when the owner discovered they were bogus.