190 ROYAL NORTON CHAPMAN 
these families, since the condition of the pelvis is a very general- 
ized one. 
Of the Jerboas (Dipodidae), the pelvis of Allactaga longior 
was examined. The symphysis is very long, one-sixth the 
length of the entire pelvis, and the pelvis is so inclined that the 
iliac axis forms a large angle with the sacral axis. The caudal 
vertebrae are free, save for a slight codéssification of the trans- 
verse processes of the first caudal vertebra with the sacrum. 
The ‘pocket rats’ (Heteromyidae), although in their sys- 
tematic relations placed very close to the pocket gophers 
(Geomydae), have a very well-developed symphysis. In the 
pocket mouse (Perognathus flavus) the symphysis is over one- 
sixth the total length of the pelvis, and in Dipodomys deserti it 
is about one-fourth the length of the pelvis. No other repre- 
sentatives of this family have been studied. 
The porcupines (Hystricidae), as found from the study of 
Hystrix, have the symphysis one-fifth the length of the pelvis, 
and the iliac axis forms a large angle with the sacral axis of the 
pelvis. The transverse processes of the first caudal vertebra 
are fused to the sacrum, but it is important to note that the 
caudal vertebrae are free from the ischium and, because of the 
pelvic inclination, dorsal to it. 
The mice (Muridae), with numerous genera with varying 
habits, present some very interesting conditions. The deer 
mouse (Peromyscus) and mice belonging to several other closely 
related genera (Phyllotis, Akodon hirtus) have been examined, 
and in all cases the symphysis is short and the pelvis is slightly 
less inclined than it is in some of the other rodents; that is, the 
pelvis approaches the horizontal position. ‘The sacral and first 
two caudal vertebrae are codssified along their transverse proc- 
esses, but the caudal vertebrae are free from the ischium and 
the pubic bones converge posteriorly. 
The meadow mice (voles, subfamily Microtinae) illustrate 
another step in the attainment of a horizontal pelvis and the 
loss of the symphysis. However, it is only a part of this sub- 
family which is so greatly modified. The musk rat (Ondatra) 
possesses a symphysis and, although the pelvis is nearly hor- 
