Turtles are diapsids of the order 'Testudines' characterized by a special bony or cartilaginous shell developed from their ribs and acting as a shield.

The smallest turtle is the speckled padloper tortoise of South Africa. It measures no more than 8 cm (3.1 in) in length and weighs about 140 g (4.9 oz). Two other species of small turtles are the American mud turtles and musk turtles that live in an area that ranges from Canada to South America. The shell length of many species in this group is less than 13 cm (5.1 in) in length.

More Info: en.wikipedia.org