  The masses of 40 neutron-rich nuclides from Z = 51 to 64 were measured at an
average precision of $\delta m/m= 10^{-7}$ using the Canadian Penning Trap mass
spectrometer at Argonne National Laboratory. The measurements, of fission
fragments from a $^{252}$Cf spontaneous fission source in a helium gas catcher,
approach the predicted path of the astrophysical $r$ process. Where overlap
exists, this data set is largely consistent with previous measurements from
Penning traps, storage rings, and reaction energetics, but large systematic
deviations are apparent in $\beta$-endpoint measurements. Differences in mass
excess from the 2003 Atomic Mass Evaluation of up to 400 keV are seen, as well
as systematic disagreement with various mass models.
