THE PELVIS OF BURROWING MAMMALS 189 
fusion of the second caudal vertebra and the ischium rather 
than through the centra of the sacral vertebrae which are 
arched dorsad. 
The iliac and sacral axes (fig. 6, Ja and Sa) are very nearly 
parallel. This is an advanced step in the attainment of a 
horizontal pelvis, in which direction Huxley (’75) believed 
mammals to be tending. In the case of the pocket gophers, 
the symphysis has moved caudad and is evidently formed by the 
ischial bones, as shown by the study of immature specimens. 
The pubic bones are nearly parallel and their ventral margins 
diverge posteriorly (fig. 6). 
This condition is undoubtedly a high specialization. By 
referring to the figure of the Monotreme pelvis (fig. 2), it will 
be noticed that this structure in the primitive mammals pos- 
sesses a2 symphysis which is very long and is formed by both 
the ischium and the pubis. In still more primitive vertebrates, 
amphibians and reptiles, morphologically the symphysis is even 
longer. 
A study of the pelvis of the rodents, in general, contributes 
instructively to the understanding of specializations such as are 
found in the pocket gophers. Apodontia of the family Apo- 
dontiidae has a relatively short symphysis. Among the ‘mole- 
rats’ (Bathyergidae), Bathyergus miratimus and Heterocephalus 
glaber were examined in the flesh. The symphysis was found to 
be very short in both cases, being a rod-like structure very much 
like that found among the pocket gophers. Since only one 
specimen of Bathyergus and four specimens of Heterocephalus 
were examined, it is not known whether there is any variation 
of the symphysis in this family. The position of the pelvic 
axes are also unknown. 
The beavers (family Castoridae) possess a long symphysis, 
and the caudal vertebrae are not codssified as they are in the 
pocket gophers. The condition in the guinea-pigs (Cavidae) is 
very similar to that of the beavers and must be considered as a 
generalized state, where the caudal vertebrae are free and a 
well-developed symphysis is present. In a morphologic way, 
the chinchillas (Chinchillidae) might also be included with 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 25, NO. 2 
