Turtles are divided into two groups, according to how they evolved a solution to the problem of withdrawing their necks into their shells. The Cryptodira (hidden neck) can draw their necks in while contracting it under their spine. The Pleurodira (side neck), now found only in fresh water environments in the Southern hemisphere, contract their necks to the side. So, the important adaptation of head withdrawing evolved twice from ancestral turtles which did not have this ability.