4.2 HIP GIRDLE CHAP. 
Ornithorhynchus comes nearest to the reptile in the fact that 
this axis is nearly at right angles to that of the sacrum. It is 
particularly interesting to find that this peculiarity of Orni- 
thorhynchus is only acquired later in life, and that the pelvis of 
the foetus conforms in these angles to the adults of other 
mammalian groups. In any case, the backward rotation of the 
pelvis is a mammalian characteristic, and it is most nearly 
approached among reptiles by the extinct Anomodontia, whose 
affinities to mammals will be dealt with on a later page (p. 90). 
Another peculiarity of the mammalian pelvis appears to be the 
cotyloid bone already referred to. In the Rabbit this bone 
completely shuts out the pubis from any share in the acetabular 
cavity ; later it ankyloses with that bone. In Ornithorhynchus 
the cotyloid or os acetabuli is a larger element of the girdle 
than is the pubis. In other mammals, therefore, it seems to be 
a rudimentary structure. But it seems to be a bone peculiar to 
and thus distinctive of the mammals as compared with other 
vertebrates. The acetabular cavity is perforated in Hehidna as 
in birds; but in certain Rodents the same region is very thin 
and only closed by membrane, as in Circolabes villosus. 
The number and the arrangement of the bones in the hind-limb 
correspond exactly to those of the fore-limb. The femur, which 
corresponds to the humerus, shows some diversities of form. The 
neck, which follows upon the almost globular head, the surface 
of articulation to the acetabular cavity of the pelvis, has two 
roughened areas or tuberosities for the insertions of muscles. A 
third such area, known as the third trochanter, is present or 
absent as the case may be, and its presence or absence is of 
systematic import. As a general rule the thigh-bones of the 
ancient types of mammals are smoother and less roughened by 
the presence of these three trochanters than in their modern 
representatives. The radius and the ulna are represented in the 
hind-leg by the tibia and the fibula. These bones are not 
crossed, and do not allow of rotation as is the case with the 
radius and the ulna. In Ungulate animals there is the same 
tendency to the shortening and rudimentary character of the 
fibula that occurs in the case of the ulna, but it is more marked. 
It has been shown in tracing the history of fossil Ungulates that 
the hind-limbs in their degree of degeneration are as a rule 
ahead of the fore-limbs. This is natural when we reflect thav 
