The whole girdle was held in position by muscles, the serrati passing from the ribs to the inner surface of the scapula and by others, sternomastoids and cleidomastoids passing from the head to the clavicular arch. Posteriorly, the coracoid is attached to the ventral surface of the abdomen.
The humerus of these reptiles has its extremities very much widened and placed nearly at right angles to one another. The articular surface of the head is screw-shaped, and fits the glenoid cavity so accurately that the bone cannot be rotated, and is restricted to a to-and-fro motion along a definite track. The widened proximal end allows the muscles which pass from the humerus to the ventral part of the animal, the pectoral and coraco-brachials, to have a mechanically favourable insertion. The widened lower end of the humerus similarly secures a favourable insertion for the flexor muscles which pass from it to the palmar surface of the hand and forearm, and take the whole weight of the anterior part of the body.
The lower end of the humerus bears a hemispherical boss on its front face, with which the head of the radius articulates, and a cylindroid articulation on its end which fits into the sigmoid notch of the ulna. The distal ends of the radius and ulna are widely separated ; they articulate with the four bones : radiale, intermedium, ulnare and pisiform, of the proximal row of the carpus. The middle row of the wrist usually consists of only two bones, the centralia, one of which forms part of the inner border, whilst the other separates the intermedium from the distal row. This consists of five bones, of which the fourth, which articulates with the ulnare, is the largest. The metacarpals articulate directly with the corresponding carpals and the number of phalanges is 2, 3, 4, 5, 3 respectively. This ensures that the ends of the fingers lie in a straight line at right angles to the animal, when the hand is placed on the ground.
The pelvis is attached to the vertebral column by the sacral ribs, which vary in number in Lower Permian reptiles from one to four. It consists of three pairs of bones, the ilia, pubes and Ischia. These meet in a triradiate suture, so that each supports about one-third of the acetabulum, with which the head of the femur articulates. The pubis and ischium of the same side are firmly united by a continuous suture and the two halves of the pelvis articulate continuously, so that the whole structure is usually described as plate-like. The femur is a straight bone,
with the articular face of its head placed on the end of the shaft. The condyles at the distal end are only slightly curved, well separated and placed at such an angle as to suggest that the knee could neither be extended into a straight line nor flexed beyond a right angle.
The tibia is a bone with an expanded upper end, and is always shorter than the fibula, which is unusually massed. At their lower ends the two bones are widely separated. The tarsus, in one case (Seymouria) has a proximal row of three bones, the tibiale intermedium and fibulae, corresponding with those of the carpus, but in nearly all other reptiles and their descendants, the mam mals and birds, the intermedium is no longer found as an inde pendent bone, even during development, and has fused with the tibiale to form an astrazalus. As a result of this fusion the tibia articulates only with the astragalus, whilst the fibula impinges on both astragalus and fibulare or calcaneum. The calcaneum is always in direct contact with the fourth and fifth (if present) distal tarsal, whilst the astragalus is separated from the first three distal tarsals by a row of two or more, usually one, centrale, the mammalian navicular. There are primitively five digits, the fourth being the longest. The phalangeal formula is 2, 3, 4, 5, 4.
The great majority of the changes which take place in the structures of these limbs during the evolution of the reptiles can be explained by a consideration of the mechanics of the structures under modified conditions of locomotion.
In the line of the mammal-like reptiles, and also in some of the other forms, the first change which takes place in the shoulder girdle is the addition of a bone, the coracoid, to the two existing in the primary shoulder girdle. These animals then acquire a pectoral girdle which resembles the pelvis in that the facet for articulating the proximal bone of the limb is carried almost equally on three bones. The glenoid cavity at first retains its screw shape, the humerus being restricted in its motion to an excursion along an arc lying nearly parallel to the ground.