Defects of the lower extremity are similarly classified to those of the upper, and have similar characters. We have Apus, Peropus, Monopns. In the last mentioned the absence of the leg may be associated with defect of the pelvis and protrusion of the abdominal contents. In addition to this we have the following special form. Siren - malformation (Sympus: Symelid). - In this form the lower extremities are coalesced, and the body is prolonged into a tapering single limb, so that there is some resemblance to the legendary siren with a fish's tail (see Fig. 14). The defect arises at an early period of development, and there is, besides the deformity of the legs, defect of the pelvis and of the intestinal and urino-genital organs. It is probable that in this case, as suggested by Dareste, the amnion by undue impingement on the caudal extremity of the embryo interferes with the growing structures. The interference is in the middle line and occurs at the time when the limb-girdle, which is to form the pelvis, is first formed, which is about the fourth week. The result of the interference is shown in the pelvic bones. The ischial and pubic bones are defective below (see Fig. 15), and their proximal parts coalesced, so that the two ischia form a transverse bar across the pelvis, and the two pubic bones project forward. By this coalescence the acetabula are approximated behind, and may coalesce into one. From the fact of the acetabula being approximated behind, the femora are brought together by their outer aspects, and coalesce by these (see Fig. 16), which are fixed in the middle line. This explains the peculiar position of the coalesced limb, which has the knee with the patellae looking backwards and flexed forwards, and has the foot with the plantar surface facing forwards and the great toe outwards (see Fig. 14).

Fig. 14. - Sympus monopus or Siren-malformation.

Fig. 15. - Synipus. Pelvis seen from the front. The ischial bones coalesced across the floor. The pubic bones coalesced and projected forwards.

Fig. 16. - Synipus. Bones of pelvis and leg, from behind. Femur, with two heads and two lower extremities. Glenoid cavities close together behind.

Various degrees of the deformity are met with according to the degree of coalescence of the limbs and of suppression of the parts which meet in the middle line. The foot may be absent (Synipus-apus), or there may be one foot at the extremity of the combined leg (Synipus monopus). The single foot is variously supplied with toes - the two outer are always the great toes, and the number of others is various - or there may be two distinct feet with the full complement of toes (Sympus dipus). In this last form the other constituents of the limb will also be less united, and there may be two femurs, but the soft parts are coalesced.

In the siren there is always defect of the external organs of generation, which are often absent altogether. The anus is usually absent, although not so in the case figured above. There are also sometimes defects in the intestine in its course.