Class Chondrichthyes (which strangely came from an earlier group of bony fishes). Some recent classification schemes instead call this a class Elasmobranchii. Most of these are marine fish, ie they live in salt water. “Cartilaginous” fish or “Elasmo branches” include sharks as well as skates, skates and chimeras (rat fish): those that paint real bones. Instead, they have a skeleton of cartilage that is reinforced by calcification. Sharks can only swim forward. This is contrary to bony fish. Many sharks have to keep swimming to breathe.
Cartilaginous fish have, among other things, a unique way of attaching teeth and dentures. The teeth are arranged in many rows, which are constantly thrown off and replaced. In contrast to bony fish, whose males and females throw their sperm and eggs into the water for mixing, these fish have an internal fertilization. “Chondrichthyan” have organs known as parentheses. This introduces a modified portion of the pelvic fins, allowing them to be introduced into female animals during copulation. Compared to bony fish mature Chondrichthyans.