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SERPENTS.
155
as the ribs move in pairs, the scutum under each pair is carried along with it. This scutum, by its posterior edge, lays hold of the ground and becomes a fixed point from whence to set out anew. This motion is beautifully seen when a snake is climbing over an angle to get upon a flat surface. When the animal is moving, it alters its shape, from a circular or oval form to something approaching a triangle, of which the surface on the
SKELETON OF BOA.
ground forms the base. The Coluber and Boa having large abdominal scuta, which may be considered as hoofs or shoes, are the best fitted for
