There are a lot of different ideas about how Helicoprion might have looked. The "buzz saw" could have been in the jaw, and that's at least what I think. It also could have been anywhere on the entire body. Helicoprion lived Carboniferous to early Triassic. The name is pronounced hella-cuh-PRY-on.
The Helicoprion in the picture below may have helped it smash ammonite shells, or any other prey with a shell, including trilobites and other shelled animals. It may look like Sarcoprion, but Sarcoprion only has half a "buzz saw," and this idea of Helicoprion has a whole "buzz saw," but half of it is wedged into the inside of its jaw.
The Helicoprion in this last picture may have opened its mouth and let its "buzz saw" hang down while swimming through a school of fish. That way, it could kill many fish and easily just swim around in the school and eat freshly killed fish.