MAMMALIA, 



471 



The pelvic arch in mammals is fairly constant in its 

 structure. The ilium always becomes firmly attached to 

 one or more of the vertebrae. It always slopes backwards 

 from its junction with the sacrum to the acetabulum, 

 whereas the acetabulum is usually immediately below the 

 ilium in reptiles. The Monotremata in this respect approxi- 

 mate to the reptiles, the angle between the axis of the ilium 

 and that of the sacrum being less acute. 



Fig. 325. — Lateral Views of — A, Crocodile's Pelvis ; 

 B, Pelvis of Prototheria ; and C, that 



OF EUTHERIA. 



a b Ilium. Ilium. 



Epipubis. Pubis. Epipubis. 



rt = Perpendicular axis through acetabulum. <^= Perpendicular axis through sacrum. 



In mammals the pubes unite with the ischia on each 

 side and thus enclose a large hole or foramen, the obturator 

 foramen. In most the pubes meet across the middle line 

 to form a symphysis pubis and the ischia also meet to form 

 an ischial symphysis, but in several types {e.g.., man) the 

 ischia no longer meet across the middle line, the pubes 

 forming the whole symphysis. A small acetabular bone is 

 also very generally present and usually fuses with one of 

 the other elements. 



In Metatheria and Prototheria there is a pair of epipubic 

 bones running forwards from the pubes, which serve, at least 

 in the former, for support of the pouch. Similar epipubic 

 bones are found in certain reptiles {e.g.., Hatteria). 



Sternum and Ribs. — The sternum in mammals arises 

 from the fusion of the distal extremities of the ribs and is 

 usually segmented into a series of joints or so-called 



