THE INTERNAL SKELETON, 195 
The ilium may be considered as answering to the scapula of 
the thoracic girdle, in spite of the extreme difference of its shape 
from that of the blade-bone of the shoulder *. It is of enormous 
size compared to that of Mammals, being greatly extended both 
in front of and behind the acetabulum. It is the bone which 
anchyloses on either side with the many vertebre which go to 
form the “ sacrum” as already described f. 
The ilium of one side of the body may so anchylose with that 
of the other side and with the sacrum that the dorsal hinder 
part of the skeleton of a bird’s trunk presents an expanded 
bony shield like that presented by the sternum on the ventral 
anterior part of the trunk skeleton. Moreover, the appearance 

Purvis or A Fown (after Parker). 
i, il, Ilium ; zs, ischium; pd, pubes ; d/, dorso-lumbar vertebrae ; cd, caudal 
vertebrxe, at the distal end of which is (py) the pygostyle t ; am, ace- 
tabulum. 
of a median ridge is more or less produced by lateral depressions, 
though there is never anything really like the keel of the ster- 
num, and for a very good reason, as we shall see when the 
muscular structure of a bird comes to be described. 
The ilium forms the upper margin of the acetabulum, and at 
the hinder part of that margin develops a strongly marked, 
somewhat flattened process bearing an articular surface. This 
is called the antitrochanteric process. 
* See ante, p. 189. 
t See ante, p. 173. t See ante, p. 175. 
O02 
