In the MONOTREMATA the pelvis is short and broad, and the pubes and ischia meet in a long symphysis. The acetabulum is perforated in _Echidna_ as in birds, but not in _Ornithorhynchus_. A pair of elongated slender bones project forwards from the edge of the pubes near the symphysis; these are sesamoid bones formed by ossifications in the tendons of the external oblique abdominal muscles, and are generally called _marsupial bones_. In the MARSUPIALIA the ilia are generally very simple, straight, and narrow, while the pubes and ischia are well developed and meet in a long symphysis. Marsupial bones are nearly always prominent, but are not developed in _Thylacinus_ or _Notoryctes_. The ischium often has a well-marked tuberosity and in Kangaroos the pubis bears a prominent pectineal process on its anterior border close to the acetabulum. The pelvis in _Notoryctes_ differs much from that in all other Marsupials, the ilium and ischium being ankylosed with six vertebrae in a manner comparable to that of many Edentates. In the EDENTATA the pelvis is generally well developed, but the symphysis is very short. In the Sloths the pelvis is rather weak and slender, the obturator foramina are very large and the ischia do not meet in a symphysis. In the Megatheriidae the pelvis is exceedingly wide and massive, and is firmly ankylosed with a number of vertebrae. In the Armadillos, Glyptodonts, Anteaters, and Pangolins it is much developed and firmly united to the vertebral column by both the ilia and the ischia. In _Orycteropus_ however the ischium does not become united to the vertebral column, and the pubis generally has a strongly developed pectineal process. In the SIRENIA the pelvis is quite vestigial. In the Dugong it consists on each side of two slender bones, one of which represents the ilium and the other the ischium and pubis; the two bones are placed end to end and are commonly fused together. The ilium is attached by ligament to the transverse process of one of the vertebrae. In the Manatee each half of the pelvis is represented by a triangular bone connected by ligaments with its fellow and with the vertebral column. In neither Manatee nor Dugong is there any trace of an acetabulum but one can be made out in _Halitherium_. In the CETACEA the pelvis is even more vestigial than in the Sirenia, consisting simply of a pair of small straight bones which probably represent the ischia, and lie parallel to and below the vertebral column at the point where the development of chevron bones commences.