A brief descr. of the locomotor apparatus, condensed from a very extensive investigation, is followed by an analysis of the 3 principal types of ophidian locomotion the horizontal undulatory movement, the sidewinding, and the caterpillar movement. The first one, the typical serpentine motion, is produced by the progression of horizontal transverse waves from the head to the tail. Progression requires firm, though small, areas of lateral resistance, against which the flanks of the snake press, the points of inflexion of the snake''s curves, midway between 2 successive vertices, being most important. There is no tractive effort exerted by the ventral surface. Side-winding is applied by viperide snakes as adaptation to locomotion on sand. It is principally like the rolling of a helix. The caterpillar movement by which the thick bodied Viperidae and Boidae can glide slowly, stretched out fully, consists of an alternate movement of the skin on the body and of the latter within its tube of skin, proceeding rhythmically craniocaudad.