. Animal physiology. Physiology, Comparative; Physiology, Comparative. 76 GENERAL STRUCTURE OF REPTILES.—TORTOISES, LIZARDS. plastron) which is often incomplete, is formed by a similar ex- pansion of the sternum or breast-bone, which is here spread out sideways, instead of being raised into a projecting keel, as in Birds. The accompanying figure will show the general con- struction of the skeleton of this tribe, the sternum being re- moved. As in the preceding figures, vc are the cervical vertebra, and vd the dorsal vertebrae ; c are the ribs extended in width, so as to unite at their edges; c
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 76 GENERAL STRUCTURE OF REPTILES.—TORTOISES, LIZARDS. plastron) which is often incomplete, is formed by a similar ex- pansion of the sternum or breast-bone, which is here spread out sideways, instead of being raised into a projecting keel, as in Birds. The accompanying figure will show the general con- struction of the skeleton of this tribe, the sternum being re- moved. As in the preceding figures, vc are the cervical vertebra, and vd the dorsal vertebrae ; c are the ribs extended in width, so as to unite at their edges; cs are the bony pieces which con- nect these ribs with the sternum; 0, is the scapula or blade- bone, here very narrow, as in birds; c/, the clavicle; co, the additional clavicle, as in birds; &, the bones of the pelvis which support the lower limbs; /, the thigh-bones; p and t, the bones of the leg. Although the bones of the toes are separate, they are enclosed in a single horny casing; this is flattened in the aquatic turtles, and forms a paddle ; whilst in the land tortoises it forms a stumpy foot. The carapace and plastron are covered with large horny plates, variously arranged in the different species, and constituting what is com- monly called tortoise-shell. These plates are often very beautifully disposed, form- ing a kind of tesselated pavement; as in the common Tortoise, which is often pre- served alive in our gardens. 93. In the tribe of Lizards, the body has no such covering ; but these animals, having more activity than the tortoises (which are proverbially slow) are enabled to make their escape from danger, whilst the latter are obliged to trust to their bony casing for protection from it. In their general form, the lizards approach the Mammalia, being all four- footed, and living for the most part on land ; but they differ from them in the very important particulars already mentioned, as well as in several others of less consequence. In general, their bodies
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