the progressive Motion of Snakes , 167 
ribs rest ; and 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 cir- 
cular or oval form, to something approaching to a triangle, of 
which the surface on the 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 
this kind of progressive motion ; there is, however, a similar 
structure of ribs and muscles in the anguis and amphisbeena. 
In the anguis the ribs are proportionally weaker, and as these 
have nothing to correspond with the scuta, it is probable this 
mode of progressive motion is less necessary to them. 
The rings of the amphisbaena are a near approach to the 
large scuta. 
DESCRIPTION OF THE PLATES. 
Plate IV. 
A lateral view of the muscles of the boa constrictor. 
A A. The straight muscles of the back. 
BB. The first set of muscles which arises from the trans- 
verse process of each vertebra, and is inserted into the rib 
behind it close to its head. 
C C. The second set. 
DD. The third set. 
EE. The fourth set. 
F F. The fifth set. 
G G. Short muscles which pass from cartilage to cartilage,. 
