Terrapin - or gland?

My terrapin is having a little trouble shedding some of the loose shell on his back and my vet suggested that I help sc it off. As I was doing this today, he got a little distressed, so i put him back in the water, when I did, I noticed a rather large flat dark green bit protruding from his backside!!! I think it may have been his , but I have also heard that terrapins have an gland in their backside that they use when the feel defensive. Anyone have any suggestions as to whether my terrapin is defensive or horny??? I can't find pics on the net to identify what it was.