General Terms
Chelonian: a member of the order of reptiles called Chelonia (also sometimes known as Testudines), made up turtles.
Turtle: any aquatic or terrestrial member of the order Chelonia.
note: the British usage of turtle often refers solely to sea turtles.
Tortoise: common term for primarily terrestrial turtles.
note: the British usage of tortoise encompasses both freshwater and terrestrial turtles.
Terrapin: common term for largely aquatic turtles, particularly in the British usage of the term. More specifically the term terrapin refers to the diamondback terrapin, Malaclemys terrapin.
Side-necked turtles: group of turtles which lack the ability to pull their heads back into their shells; they retract their heads under their shells by turning them sideways under the margin of their shells.
Anatomy
Scute: large scale like structures made up of keratin (like fingernails) that cover the shells of turtles. They do have nerve endings, and the turtle can detect something touching its shell. Turtles can regenerate damaged scutes, and on some turtles they have rings similar to growth rings on trees and can be used to estimate age.
Plastron: The ventral (bottom) portion of a turtle's shell (under the abdomen/chest). It is made up of bony plates and covered with scutes. In some turtles, the some of the bony plates making up the plastron are hinged.
Carapace: The dorsal (top) portion of the turtle shell (i.e. over the back). The carapace is also made up of bony plates, that are fused to the ribcage and backbone of the turtle. The seams of the bony plates do not necessarily coincide with the seams between scutes.
Cloaca: common opening of the digestive, urinary and reproductive tracts.
