Tracing the evolution of the pelvis, it is to be found first in fishes where it is a loose disjointed, variable structure not attached to the spine; in some it consists merely of ischia. In reptiles the box assumes a form suggesting higher types. Passing from the toads to turtles and to the crocodiles, the elements which go to make up the pelvis assume a more and more osseous character and become more and more definitely articulated with the spine or with the differentiated sacrum. Its purpose in these low forms is mostly as a support for the muscles of the legs and back. Birds present an advance in pelvic construction but with a great preponderance of the vertebral column since thirteen to seventeen bones may fuse to form a sacral “roof.” The ilia, ischia and pubis are firmly combined. The box is long and narrow with a heavy part for the acetabulum and broad surfaces for muscular insertions. The pelvis of birds differs from that of cold-blooded vertebrates in the greater number of vertebral segments entering into its composition, and in their bony confluence. It differs from that of mammals by being unclosed by an anterior symphysis and by a widely perforate acetabulum. The ossification of the pelvic bones is to afford a support for the legs, and the open pelvis allows passage of the large brittle egg. The shape of the pelvis is of little importance in parturition in the foregoing animals, except for birds which bear large eggs when the pubis anteriorly is open for that purpose. In monotremes one finds the reptilian type of pelvis with the three divisions of the innominate bone remaining separate. The pelvis is short, heavy and flat and is fitted with marsupial bones. The marsupial pelvis, possessing parallel walls made by the ilia and ischia and a long symphysis, resembles roughly a triangular prism. The sacrum is wedge- shaped, without a promontory and has a considerable movability. The marsupial bones are quite long. The triangular outlet is many times the size necessary for the passage of the fetus. The rodent pelvis is difficult to summarize morphologically by reason of the great diversity of form occurring in the many genera of the order, but it may be said that here the pelvis is usually of a type rather higher in the scale than the other structural characteristics of the order would indicate. The outlet is more commonly ovate than rectangular, the ischia and ilia lie at a more marked angle to each other, and the true conjugate forms a lesser angle than is common in quadrupeds. The sacroiliac joint is, in general, freely movable.