42 HIP GIRDLE 



Ornithorhynchvs 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 tliis peculiarity of Orni- 

 tlhorliynclius 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 

 aflinities to mammals will be dealt with on a later page (p. 90). 

 Another peculiarity of the mammalian pelvis appears to be the 

 cotyloid lione already referred to. In the Eabbit this bone 

 completely shuts out the pul)is from any share in the acetabular 

 cavity ; later it ankyloses with that l)one. In Ornithorhynchus 

 the cotyloid or os acetaliuli 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 Echidna as 

 in birds ; but in certain Eodents the same region is very thin 

 and only closed by membrane, as in Circolahes villosus. 



The numl^er 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, know^n 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, l)ut 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 that 



