THE PELVIS OF BURROWING MAMMALS 187 
Thanks are due Dr. H. D. Reed, under whose direction this 
work was done, for many suggestions and for assistance while 
the work was in progress. Acknowledgments are also due the 
authorities of the Smithsonian Institution for placing at the 
disposal of the author the material in the National Museum, 
especially Drs. Leonhard Stejneger and Gerrit S. Miller and 
Mr. Ned Holister for kindnesses and assistance while examining 
material in the Smithsonian collection. 
THE STRUCTURE OF THE PELVIS 
1. The pelvic bones of the Rodentia 
The two types of pelvis which occur among the pocket gophers 
(family Geomydae) are well illustrated by Geomys bursarius 
(figs. 4, 5, 6, and 7). It will be noticed that these two pelves 
differ not only in the presence or absence of the symphysis, but 
in that a triangular portion of bone lying at the sides of the 
symphysis in the pelvis of the closed type is absent in that of 
the open type. These two types of pelvis occur not only in the 
same species, but also in the same sex, as shown by the females 
where both types occur. All the males examined possess the 
symphysis. 
No intermediate conditions have been found; the bones either 
meet to form a symphysis or are widely separated. Very young 
females have been found possessing a symphysis, while old 
females without a symphysis are not uncommon. It seems, 
therefore, that the presence or absence of the symphysis cannot 
be a matter of the ossification of the bones due to age. 
The following table shows the proportion of the open and 
closed pelves found in the species of the pocket gophers which 
have been examined. Since the symphysis was present in all 
of the males examined, only the females have been included in 
the table. 
