ORDER III.—_Chelonia._[8] This name is derived from the Greek word for tortoise. The class includes all the varieties of tortoises or turtles. Tortoise means crooked, or twisted, and refers to the awkward manner in which the limbs are twisted about when in motion. There are two skeletons, the one external, including the viscera and the whole muscular system, which is internally attached to it. The upper plate of this covering, called the carapace, is more or less convex, and made up of strong elastic plates. The lower, or plastron, is flat and smooth, being apparently an extension of the sternum or breast bone. These plates are joined together at their lateral edges, leaving anterior and posterior openings through which the head and limbs are extended. When at rest, these are usually under the covering. The head and neck are covered with a rough, corrugated skin, and the horny upper jaw terminates in a strong, hooked beak. They are usually distinguished as marine, fresh water, and land turtles. The former are good swimmers, but their movements on land are slow and awkward, and if turned on their backs they are quite helpless. Some marine turtles are very large, being from four to six feet in diameter, and weighing from 600 to 2,000 pounds. Usually they weigh much less. All are oviparous, the female producing about one hundred and fifty hard-shelled eggs at a time. For this purpose she cautiously comes ashore, and, dextrously using her hind flippers, excavates a hole in the sand, from a foot to eighteen inches in depth, lays her eggs, and, having carefully covered, leaves them to be hatched by the heat of the sun. The green, edible turtle is very valuable, and furnishes much wholesome and delicious food. The “hawks-bill” is seldom used for food, but supplies the greater part of the well known “tortoise shell” of commerce. There are many other species, both marine and fresh-water, whose flesh or shells are of considerable value. Soft Tortoises, Snappers, Mud-Turtles, and Terrapins, all have some peculiar characteristic.