C. THE PELVIC GIRDLE. The pelvic girdle of the Frog is much modified from the simple or general type found in the Newt (p. 149). It is a V-shaped structure consisting of two halves which are fused together in the middle line posteriorly, while in front they are attached to the ends of the transverse processes of the sacral vertebra. Each half bears at its posterior end a deep cup, the =acetabulum=, with which the head of the femur articulates. Each half of the pelvis ossifies from two centres. The anterior and upper half of the acetabulum, and the long laterally compressed bar extending forwards to meet the sacral vertebra ossify from a single centre and are generally called the =ilium=; it is probable however that they represent both the =ilium= and =pubis= of mammals[58]. The posterior part of this bone meets its fellow in a median symphysis. The posterior third of the acetabulum is formed by a small bone, the =ischium=, which likewise meets its fellow in a median symphysis. The ventral portion of the pelvic girdle never ossifies, even in old animals being formed only of calcified cartilage. This is generally regarded as the pubis, but it perhaps corresponds to the =acetabular bone= of mammals. D. THE POSTERIOR LIMB. This corresponds closely to the anterior limb and, like it, is divisible into three parts, the =thigh=, the =shin= or =crus= and the =pes=. As was the case with the anterior limb, all the long bones have their ends formed by prominent epiphyses which do not unite with the shaft till late in life. In the =thigh= there is only a single bone, the =femur=. The =femur= is a moderately long, slender bone with a well-ossified hollow shaft slightly curved in a sigmoid manner. Both ends are expanded, the proximal end is hemispherical and articulates with the acetabulum, the distal end is larger and more laterally expanded. The =shin= likewise includes a single bone, the =tibio-fibula=, but this, as can be readily seen by the grooves at the proximal and distal ends of the shaft, is formed by the fusion of two distinct bones, the =tibia= and =fibula=. The tibio-fibula is longer and straighter than the femur. The =pes= consists of two parts, the =ankle= or =tarsus= and the =foot=.