298 DISEASE IN WILD MAMMALS AND BIRDS 
merely of iscliia. In reptiles the box assumes a form 
suggesting higher types. Passing from the toads to 
turtles and to the crocodiles, the elements which go to 
make up the pelvis assume a more and more osseous 
character and become more and more definitely articu- 
lated Mith the spine or with the differentiated sacrum. 
Its jDurpose in these low forms is mostly as a support 
for the muscles of the legs and back. Birds present an 
advance in pelvic construction but with a great prepon- 
derance of the vertebral column since thirteen to 
seventeen bones may fuse to form a sacral ' ' roof. ' ' The 
ilia, ischia and pubis are firmly combined. The box is 
long and narrow with a heavy part for the acetabulum 
and broad surfaces for muscular insertions. The pelvis 
of birds ditfers from that of cold-blooded vertebrates 
in the greater number of vertebral segments entering 
into its composition, and in their bony confluence. It 
differs from that of mammals by being unclosed by an 
anterior symphysis and by a widely perforate acetab- 
ulum. The ossification of the pelvic bones is to afford 
a support for the legs, and the open pelvis allows passage 
of the large brittle egg. The shape of the pelvis is of 
little importance in parturition in the foregoing animals, 
except for birds which bear large eggs when the pubis 
anteriorly is open for that purpose. 
In monotremes one finds the reptilian type of pelvis 
with the three divisions of the innominate bone remain- 
ing separate. The pelvis is short, heavy and flat and is 
fitted with marsupial bones. The marsupial pehds, 
possessing parallel walls made by the ilia and ischia 
and a long symphysis, resembles roughly a triangular 
prism. The sacrum is wedge-shaped, \\T.thout a promon- 
tory and has a considerable movability. The marsupial 
bones are quite long. The triangular outlet is many 
times the size necessary for the passage of the fetus. 
The rodent pelvis is difficult to summarize morpho- 
logically by reason of the great diversity of form 
