Turtles and Tortoises
Turtles are easily recognized by their shells and have not changed much in the past 200 million years. Turtles can be classified in three basic categories. Turtles generally spend most of their time in the water. Terrapins generally spend an equal amount of time in and out of the water. Tortoises spend a majority of their time on the land. All turtles do not have teeth but have a beak called a tomia instead.
The shell has two basic parts, the carapace and the plastron. The carapace is the top shell and the plastron is the shell covering the belly. The shell consists of three layers. First there is a layer of bone which is actually modified ribs and vertebrae that have fused to form the carapace and an enlarged sternum which forms the plastron. Above the bone is a thin layer of tissue containing blood vessels and nerves. Finally the top layer consists of modified scales called scutes. The exception to this are the soft shell turtles that have a thick leathery skin instead of bone on the carapace.