MAMMALIA. 47 
The pelvic arch in mammals is fairly constant in its 
structure. The ilium always becomes firmly attached to 
one or more of the vertebrz. 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 Monotremaza 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 6 IMQlium. Ilium @ 36 llum, a@ & 
f 
Ischium. 
Epipubis. Pubis. Epipubis, 
a= Perpendicular axis through acetabulum, 4= Perpendicular axis through sacrum. 
In mammals the pubes unite with the ischia on each 
side and thus enclose a large hole or foramen, the od¢urator 
JSoramen. 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 (¢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 (¢.g., Watteria). 
STERNUM AND Rips.—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 
