98 COMPARATIVE ANATOMY. 



shut out from the acetabulum by the cartilaginous pars acetabularis 

 (Fig. 80, B, t> -P), and in them, as well as in Birds, the acetabulum 

 is perforated (Fig. 80, B, &, and Figs. 81 and 82, a). 



What most distinguishes the pelvis of Dinosauria and Birds 

 from that of Reptiles is the position of the pubis (.Figs. 81 and 

 82, P 1 ). It has the form of a delicate bar of bone directed back- 

 wards, and running parallel and even fusing (Birds of flight) with 

 the ischium, which also extends far backwards. It is very difficult 

 to explain the homology of a strong bone, arising from the 

 antero-ventral border of the acetabulum of Dinosaurians, indi- 

 cated by the letter P in Fig. 81. It is, however, probably to be 

 looked upon as a special outgrowth from the pars acetabularis of 

 each side. Rudiments of this bone, which may be called the 

 pectineal process of the pubis, are also present in Birds (Figs. 

 82, # 8'3, pp). 



In Birds the elements of the pelvis usually become anchylosed 

 to one another, while in Reptiles and Dinosaurians they remain 

 distinct. The pubis of either side meets its fellow only in Struthio, 

 and the ischium in Rhea. 



The pelvis of Arcliseopteryx possessed a broad pre-acetabular and a long 

 and slender post-acetabular portion of the ilium. The ischia appear to have 

 been united by symphysis : nothing is known of the pubis. 



Mammals. The four elements of the pelvis here remain 

 separated for a long time by cartilage, but later they become fused 

 together. The angle between the axes of the ilium and sacrum 

 is smaller in Monotremes than in the higher Mammals. 



The original type with both pubic and ischiatic symphyses is 

 seen in Monotremes, Marsupials, many Rodents, Insectivores, 

 Ungulates, and Carnivores. In many other Insectivora and 

 Carnivora, and more particularly in the highest forms, the Pri- 

 mates, the ischia no longer meet below. The greatest amount of 

 variety in the form of the pelvis in any one order is seen in 

 Insectivores, in some of which (e.g. Mole), as well as in most Bats, 

 the symphysis pubis is not closed. 



There is no pars acetabularis (see Fig. 83B, A ) in Monotremes and Bats, nor 

 in numerous representatives of the other principal Mammalian groups. It 

 always lies anterior to the " incisura acetabuli," and is most strongly developed 

 relatively in the Mole (Talpa), where it shuts the ilium as well as the pubis 

 out of the acetabulum ; in by far the greater number of Mammals the pubis 

 only is thus excluded. The ischium always forms part of the acetabulum. 

 In older individuals the acetabular bone may become united with either of the 

 other three pelvic bones : thus in Man, some Rodents, and Marsupials, it 

 fuses with the pubis ; more commonly, however, it fuses with the ischium or 

 ilium. In the Pinnipedia all four elements take part in the formation of 

 the acetabulum. 



The pars acetabularis always becomes ossified much later than the other 

 pelvic elements, and the pubis ossifies later than the ilium and ischium. 



In Monotremes and Marsupials of both sexes, two strong 

 bones ("marsupial bones") (Fig. 84, Om) arise from the anterior 



