Hymenocallis latifolia) and not an amaryllis.
It's not entirely surprising that I didn't know what it was. The plant tagged along for the ride when I transplanted a huge coontie plant from a friend's yard to my own. I didn't want to try digging the bulb out of the coontie, so I let it be. More than five years later, I finally know what sort of bulb it is.
This is a good example of why the experts tell you not to rip things out when you move into a new house. If you can't be sure of a plant's identity, you never know what surprises might await you!