PELVIC GIRDLE — MAMMALS 



273 



well-developed prepubic (ilio-pectineal) process on the anterior 

 border of the pelvis, this sometimes arising from the ilium. 



Marsupials and Monotremes have a pair of marsupial bones, 

 preformed in cartilage (persisting as such in Thylacinus) and movably 

 articulated with the pubes (fig. 298); they are about equally devel- 

 oped in the two sexes. Their ontogeny is imperfectly known and 

 there is uncertainty as to their homologies. They are contradicto- 

 rily stated to arise independently in the hnea alba, and from 

 the pubic cartilages. They have been compared with prepubic 

 processes, with epipubic structures (paired in some reptiles, and 

 with the Urodelan ypsiloid cartilage. They are relatively larger in 

 Monotremes than in Marsupials, 

 although the marsupium is less devel- 

 oped in the former order. No similar 

 structures occur in Placentals, unless it 

 be the paired {Manis, Pteropus) or 

 unpaired bones (Dasypus, Cholcepus) in 

 the symphysial cartilage. 



Most mammals (Monotremes, 

 rodents and bats excepted) have the 

 pecuhar acetabular (cotyloid) bone 

 which ossifies separately in the pelvis. 

 In some mammals it remains discrete, 

 but usually it fuses with ischium or ihum, excluding the pubis from 

 the acetabulum. When it unites with the pubis, the latter appears 

 to enter the cup, which is not the case except in Monotremes, seals and 

 some Ungulates. The acetabulum is usually a hemispherical cup 

 when the limb has freedom of motion, but when it swings in a single 

 plane, the shape may be altered. The cup usually faces laterally, 

 but in bats (fig. 301) it is more dorsal and the knee looks dorsally in 

 adaptation to the wing membrane. 



Fig. 298. — Pelvis of Ornilhor- 

 hynchus. a, acetabulum; il, ilium; 

 is, ischium; tn, marsupial bone; o, 

 obturator fen£stra; p, pubis; sv, 

 sacral vertebrae. 



IxsECTR-QRA. — The Talpids (fig. 299) have a very long pelvis, all bones being 

 long and slender, and in these and the Soricidze there is no ventral symphysis. 

 The symphysis is short in Erinaceus and restricted to the pubes which are 

 ankylosed, while in Monotyphla both bones are concerned. The Talpids have 

 a long and narrow obturator foramen. 



Chiroptera have a small pelvis (fig. 300), in correlation with the slight 

 use of the hind limbs. Except in Rhinolophidse the halves are connected by 

 ligament. The ilium is a rod, the pubes have a prepubic process which, in the 



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