Gradually, as an integral part of the whole process whereby these animals acquired a more rapid and less clumsy gait, the plane of this glenoid cavity becomes twisted round, so that the humerus moves freely in a dorso-ventral direction and the elbow is no longer directed outward, but is drawn in toward the side of the body, nearly to the stage in which it exists in the more primitive mammals. This change results in the restriction of the glenoid cavity to the scapula and coracoid alone, the precoracoid no longer contributing to it.
Concurrently with this change, the humerus took up a more vertical position, so that the muscles connecting it with the cora coid could become smaller, in part because the forces they had to exert were actually reduced and, in part, because their insertion became more favourable. Thus the coracoid and precoracoid suffer steady reduction compared with the scapula. Finally, in order to secure a larger surface for its attachment, one muscle, which serves to support the animal's weight and to drive the humerus downwards, migrates on to the inner surface of the scapula, the tendon by which it is attached to the humerus passing over a notch in the front border of the scapula below the point at which the clavicle is attached to that bone. In this way a defi nite acromium becomes established, and the upper part of the anterior border of the scapula becomes recognizable as the homologue of the spine of a mammalian scapula.
The most important change in the clavicular arch is the gradual reduction and final complete disappearance of the cleithrum. The other bones sink in from their original position in the skin; so that they become surrounded by muscles on all sides, but other wise they suffer comparatively little change during the evolution of the mammals.
The great changes in the position of the fore limb during the development of the mammal-like reptiles necessitate correspond ing modifications of the structure of the humerus and other limb bones. Of these, the most striking is the gradual narrowing of the two ends of the humerus and their rotation until they become nearly parallel.
In the hand, the number of phalanges in the third and fourth fingers is reduced to three, so that the formula becomes that characteristic of mammals, 2, 3, 3, 3, 3. It is evident that this
change is associated with a new pose in which the third finger, which becomes the longest, lies parallel to the mid line of the animal and the others are symmetrically placed on each side of it.
In the pelvis, the most important changes are a widening of the upper part of the ilium associated with an increase in the number of vertebrae in the sacrum, and the development of an obturator foramen, a gap lying in the suture between the pubis and ischium.
The femur so changes its shape that it can lie with the knee directed as much forward as outward, and the lower leg become capable of much freer movement. At the same time the astragalus and calcaneum shorten so that the tibia and fibula rest partly on their upper surface, thus forming an ankle joint which is on the way to a mammalian structure and the phalangeal formula of the foot becomes reduced to 2, 3, 3, 3, 3.
The non-mammal-like reptiles exhibit so many different types of adaptation that a full analysis of the structures of their limbs is impossible; indeed, it has not yet been systematically attempted. Some of the main types of life are here discussed with reference to a particular case.