Romeo is a polydactyl cat. He has one extra toe on each front paw. These cats are common on the East Coast, so are sometimes called Boston Thumb Cats. Hemingway favored this kind of cat, so it is sometimes called a Hemingway Cat. (Thanks to Julie & Dave for this info.)
One of the many six-toed cats at the Ernest Hemingway House in Key West, Florida.
The extra thumbs are not necessarily a disadvantage; they can give the kitty extra dexterity. I believe the polydactyl cat's extra dexterity (and Hemingway's favor) will allow it to eventually take over the feline kingdom. In a thousand years, cats will have paws capable of making tools. Then, I have no doubt, the kitties will build a sophisticated material culture.
Ultra-civilized felines of the future.