1. The rectilinear mode of progression in Boa occidentalis is described. It is fundamentally similar to that of an earthworm. The terms ‘snail’ and ‘caterpillar’ principles of other authors are inappropriate.

2. During movement, waves of muscular contraction and relaxation pass in a posterior direction over the ventral cutaneous musculature.

3. The ventral surface moves forward in ‘steps’ while the vertebral column and all parts rigidly connected with it move forward at relatively constant speed. The ribs do not execute any appreciable movement relative to the vertebral column.

4. The probable interaction of the musculature, as deduced from the mechanics and the geometrical arrangements, is discussed.

5. Rectilinear progression is probably a unique type of vertebrate locomotion in so far as it does not involve any lever actions of skeletal structures.

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