In all of them a distinct neck is absent, the body is of circular section, although variable in length, and the tail is usually of considerable size.
The fore limbs were attached to the body immediately behind the head, the upper arm lies parallel to the ground, and was capable of being moved backward and forward only. The elbows were thus pointed directly outward. The forearm lay nearly parallel to the principal plane of the animal, and made a very small angle with the ground. The wrist was large in comparison with the forearm, and the hand possessed five somewhat spread ing digits. The hind leg was attached to the body at a considerably higher level than the fore leg. The thigh projected freely from the body, almost at right angles, and the lower leg made a wide angle with it, indeed the stiff knee could not, in many cases, be bent to a right angle. The ankle joint was flexible, and the five toes greatly resemble the fingers of the same animal.
As the large head makes the load carried by the fore legs rather larger than that on the hind, the hand is generally larger than the foot.
These animals, like the lizards and salamanders of to-day, threw their backbones into lateral waves as they walked. Their procedure was as follows :—When the animal is standing with its right fore leg advanced to the greatest possible extent, and the right hand on the ground, the head is turned to the left, and the left hand lies near to it but is ready to be lifted. The trunk is thrown over to the right side and the base of the tail to the left. This body flexure implies that the right hind leg is turned some what backward and the left hind leg is directed forward. The left hand is then lifted from the ground by movement at the elbow, and carried forward not only by a movement of the upper arm on the shoulder girdle and a straightening of the elbow, but also by a bending of the backbone so that the head becomes directed to the right. This movement of the back involves a corresponding twist of the pelvis, which brings the left hind leg to its backward position, and makes it necessary to lift the right foot from the ground. The right hind leg is then swung forward by motion, mainly at the hip joint, and the foot placed down as far ahead as possible. During these movements the animal, as a whole, has travelled forward and the right hand is ready to be raised. Its movements agree exactly with those of the left, and
it is followed in turn by the left hind foot.
Thus the animal progresses with a waddling gait, the head and body being constantly thrown from side to side of the line along which the animal is moving. The feet are moved one at a time, so that the animal is never standing on less than three of them, and are placed wide apart. This mode of walking must have been extremely slow and clumsy; measurements suggest that a reptile about a yard in length, without the tail, must have made a track 15 in. in width, with a stride of some 6 or 8 inches.
Fossil materials enable us to trace the steps whereby the later reptiles gradually improved their modes of walking, until on one line, they became like the more primitive mammals, walking with their bodies raised high above the ground, the feet brought in towards the middle line and the stride long, whilst along a second course they became bipedal, striding along on their hind legs, with their heads raised high in the air.
The nature of the skeleton and musculature which is associated with the primitive type of locomotion is as follows:—The shoul der girdle consists of the pair of primary elements, each of which is in the most primitive forms, Seymouria and Varanops, ossified as two bones, the dorsal scapula and ventral precoracoid. The glenoid cavity has a characteristic shape in that its articular surface is a rather narrow screw-shaped strip of a cylinder whose axis is nearly vertical. The glenoid cavity is shared nearly equally by the two bones. The two halves of the primary shoulder girdle do not touch one another in the mid line ventrally; but are held in position with respect to one another by the powerful clavicular arch. This consists of pairs of cleithras and clavicles and an interclavicle. Each cleithrum is firmly attached to the front edge, and sometimes to the upper end of the scapula. The clavicle is firmly attached to the front face of the lower end of the cleithrum and has no contact with the scapula; its lower end is turned inward so as to underlie the thorax, and is usually widened, its lower end underlying the lateral margin of the interclavicle. The interclavicle is usually a thin flat bone, with a widely ex panded anterior end, and a narrower shaft projecting posteriorly under the sternum, which is unossified.