In addition to the signs for a kosher fish mentioned in the Torah, The Tur (Y.D. 83) brings alternate kosher signs for fish that are not accepted by all Poskim. Namely, the head of a kosher fish is not sharp and the fish has a spine.
Can someone explain what these signs are? What is considered a sharp head? What is meant by a spine (I'm assuming it doesn't mean the skeleton)? Pictures would be awesome as well.
If it helps, here's the wikipedia article on Fish Anatomy