45 



is changed. It ivill then he the primary turn ivhicli 

 discharges itself against the spinal articulation, and 

 tliis gives the locomotive force, the other turn 

 merely guiding the direction. 



The primary turn would be in the reverse direc- 

 tion for a different succession of twists, \dz., such 

 as would have place fiom beginning with a rear 

 point of application, but, if we be not mistaken, 

 the ribs of the common snake are, normally, so 

 inclined as to bring the concave surface to the 

 front, when they are not in action, a circumstance 

 wliich would indicate that the final slip at the 

 articulations is always forward. We expect to 

 show how locomotion backward may be produced 

 with an anterior point of application leading, and 

 it is, we suppose, for this reason, viz. : retrogres- 

 sion being, in vertebrate animals, derived from 

 progressive action, that motion backward is some- 

 what awkward in comparison with the motion 

 forward. 



There is one species of snake, the amphishoena, 

 which, it is said, moves with equal facility in 

 either direction. Whether in these the ribs are 

 so set on that they may discharge by a slip back- 

 ward, as well as forward, and so, readily, inter- 



