154 



COMPARATIVE ANATOMY 



Mammals. The ilium and ischium of Mammals, like those 

 of the Anura and- Sauropsida, are respectively preacetabular and 

 postacetabular in position, and the elements of the pelvis remain 

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

 (Fig. 114). The pubis always takes less part in the formation of 

 the acetabulum than do the other two bones, and may be more or 

 less entirely shut out from it by an ossification of the pars aceta- 

 bularis, which subsequently unites with either the ilium, ischium, 

 or pubis (Fig. 115). This acetabnlar bone is especially well 



FIG. 116. PELVIS OF A, Echidna hystrix (ADULT), AND B, Didelphys azarce 

 (FCETUS, 5'5 CM. IN LENGTH). From the ventral side. 



Ep, epipubis (marsupial bone) ; Fobt, obturator foramen ; J, ilium ; Js, 

 ischium ; Lg and Lgt, ligament between the pubis and epipubis ; P, pubis ; 

 Sy, ischiopubic symphysis ; Tub.il.p, iliopectineal tubercle; **, cartila- 

 ginous apophysis at the anterior end of the epipubis. 



In Fig. A, GH, articulation between the pubis and epipubis ; Tb, cartilaginous 

 tuber ischii ; Z, process on the anterior border of the pubis ; t*, t, ft, ilio- 

 and ischio-pubic sutures. 



In Fig B, b, 6 1 , cartilaginous base of the epipubis, continuous with the inter- 

 pubic cartilage at t ; *, *f, ischio-pubic and ischio-iliac sutures. 



developed in the Mole, in which it shuts the ilium, as well as the 

 pubis, out of the acetabulum ; in Monotremes the acetabulum is 

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

 is largest in Ornithorhynchus, and most acute in Rodents ; the 

 ilium is connected with a varied number of vertebrae in the 

 different forms. 



The original type with both pubic and ischiatic symphyses, 

 indicating an elongated form of pelvis, is seen in Monotremes, 

 Marsupials (Fig. 116), many Rodents, Insectivores, and Ungulates. 

 In many other Insectivores, in Carnivores, and more particularly 



