Multiple Mole Thumbs

Moles have polydactyl forepaws, meaning they have an extra thumb next to the regular thumb.

Picture from Wikipedia

 While the mole's other digits have multiple joints, the extra thumb has a single, sickle-shaped bone which develops later and differently from the other fingers.  This supernumerary digit is species-specific, as it is not present in shrews, the mole's closest relatives.