And maggots equals toxins.Looks to me as if it was bumped by a car and made airborne, then landed hard and skidded on it's carapace. I've never had occasion to use it, but I've heard good things about the healing powers of silver sulfadene cream. See if you can get either a prescription from your vet or if he'll sell you some. I would worry about internal damage.
Soaking in warmish water always makes them feel better.
As was already said, keep flies off it so they can't lay eggs on the wound (eggs=maggots in just a day).