I believe unless you sign the back of the ticket, it's not legally yours. Chances are if someone else found it and tried to redeem the prize, you could take them to court and tie the prize up for long enough that the person who found the ticket would offer to settle however that's going to be about it.
There was a similar case in Boston a couple of years ago ... older woman would call her lotto bet into the convenience store down the street. She played the same numbers every week and finally won. The convenience store owner then cashed the winning ticket himself claiming that because he placed the bet and she hadn't paid for the ticket that it was his. Courts eventually ruled in favor of the woman.