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 circular 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 amphisbæna.

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 amphisbæna are a near approach to the large scuta.

DESCRIPTION OF THE PLATES.

PLATE IV.

A lateral view of the muscles of the boa constrictor.

AA. 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.

CC. The second set.

DD. The third set.

EE. The fourth set.

FF. The fifth set.

GG. Short muscles which pass from cartilage to cartilage.

HH. A set of oblique muscles which passes from the anterior side of the bony extremity of each rib to the posterior edge of each scutum.

II. Muscles which pass from the ribs near their heads obliquely backwards, to be inserted into the skin at the edge of each scutum.

K. Muscles of the scuta.

PLATE V.

An internal view of the abdominal muscles of the boa constrictor.

A A. The muscles which pass from cartilage to cartilage of the different ribs.

BB. A set of muscles which passes from the point of each rib over two ribs to the middle of the third.

CC. A similar set of muscles continued from the opposite side of the rib, passing over three ribs to the body of the vertebra.

DD. The abdominal muscles which arise from the anterior edge of each rib, and pass to the linea alba.

EE. The linea alba.

FF. The termination of the set of oblique muscles which passes from the bony extremities of the ribs to the edges of the scuta.

GG. The muscles of the scuta consisting of two sets which decussate each other.

PLATE VI.

Represents two vertebræ and portions of ribs of the large boa to show their articulating surfaces.

aa. The process peculiar to the vertebra of the boa.