  This paper introduces a new two-parameter family of dwarf spheroidal (dSph)
galaxy models. The density distribution has a Plummer profile and falls like
the inverse fourth power of distance in projection, in agreement with the
star-count data. The first free parameter controls the velocity anisotropy, the
second controls the dark matter content. The dark matter distribution can be
varied from one extreme of mass-follows-light through a near-isothermal halo
with flat rotation curve to the other extreme of an extended dark halo with
harmonic core. This family of models is explored analytically in some detail --
the distribution functions, the intrinsic moments and the projected moments are
all calculated. For the nearby Galactic dSphs, samples of hundreds of discrete
radial velocities are becoming available. A technique is developed to extract
the anisotropy and dark matter content from such data sets by maximising the
likelihood function of the sample of radial velocities. This is constructed
from the distribution function and corrected for observational errors and the
effects of binaries. Tests on simulated data sets show that samples of 1000
discrete radial velocities are ample to break the degeneracy between mass and
anisotropy in the nearby dSphs. Interesting constraints can already be placed
on the distribution of the dark matter with samples of 160 radial velocities
(the size of the present-day data set for Draco).
