PELVIC GIRDLE — MAMMALS 271 



often a process from the ischium fuses with the anterior part of the 

 pubis, bounding an obturator foramen (fig. 295, A), and behind this 

 another process from the ischium may approach and even fuse with 

 the posterior part of the pubis (fig. 295, B) enclosing an ischio-pubic 

 fenestra. 



The short ilium of Archaopteryx is not fused with the true sacrals. The four 

 anterior synsacrals of penguins are fused and connected by ligament only with 

 the ilium, while a prepubic process arises some distance in front of the acetab- 

 ulum. The anterior part of the ilium is the longer in Raptores (fig. 295, A) 

 and the pubis is greatly reduced, scarcely extending behind the obturator 

 foramen. 



Fig. 296. — Pelvis of Strulhio. a, acetabulum; il, ilium; is, ischium; o, obturator 

 part of ip, ischio-pubic fenestra; p, pubis; pp, prepubic process; v, vertebral column. 



Ratites show several primitive characters. Struthio has a pubic symphy- 

 sis, and except in Rhea and Dromceus, the hinder end of the ischium is free from 

 the ilium. The two ilia of Rhea fuse ventral to the posterior end of the synsacrum. 

 Prepubic processes are usually well developed, but in adult Carinates are often 

 lacking, although present in the embryo. 



MAMMALIA. — A pelvis occurs in all mammals and is well 

 developed except in Cetacea and Sirenia. In all, except these two 

 orders, it is composed of the three bones (an acetabular bone is also 

 often present), those of each side fusing early, except in Monotremes, 

 to an innominate bone. Usually the two innominates are not fused 

 in the mid-ventral line, and they may even fail to meet (many 

 Insectivores, etc.). 



In ontogeny the procarfilage of either side has three radiating 

 processes, iliac, ischiadic and pubic, the obturator nerve passing 

 between pubis and ischium. Later, a process from the distal end of 

 the ischium (fig. 297) meets and fuses with the distal end of the 

 pubis, closing the ischio-pubic fenestra (obturator foramen of human 



