i've had both the common and AST...that is a common...even when small, AST shells have a more elongated boxy look, they are more flat then round, and the spikes stick up alot higher then in you pic...an AST can't put its head in its shell and it's neck is super short, and the have a hooked upper beak... common snappers necks are extermemtly long and when they put there head in there shell, the excess fat plops out, their mouth is shaped differently then an AST, and they don't have the wormy tounge...still quite a catch though. i had mine from 1in. to 15in. (thats just the tip of the tail to the tip of his shell, was to afraid to measure it with his head and neck fully extended) he was so massive and scary, i had to give him away!!! i couldn't clean the tank, and believe me, they are very, very messy!!! my AST got even bigger, but he rarely moved around the tank, he used his worm like tounge to catch prey, the common charges it and enhales prey.