The primary difference between the three reptiles has more to do with where they live than actual physical differences. In fact, use of the three terms varies in different parts of the world. In the United States, all freshwater, marine, and most land testudines are known as turtles. The species of testudines found only in the brackish waters of marshes and river inlets along the coast is called a terrapin. Totally terrestrial testudines are tortoises.
Turtles are the most ancient of all living reptiles.
They first appeared on earth some 200 million years ago and have remained relatively unchanged since that time. The shell is actually fused bone, consisting of the turtle's rib cage and spinal cord. The shells of water turtles are much flatter and more streamlined than land turtles offering less water resistance during swimming. All aquatic turtles have webbed feet which help them displace water with each stroke of their legs. More than 200 modern species of turtles live in the warmer parts of the world, populating every continent except Antarctica and occurring in every ocean except the Arctic.