  We present an analysis of a mosaic of ROSAT PSPC and Einstein Observatory IPC
X-ray observations of 14 clusters and 2 groups of galaxies, enclosed in a sky
area 15 degrees by 20 degrees centred on A3558 within the Shapley Supercluster
region. From the mass of each cluster, extrapolated to a density contrast of
500, we define 4 large structures: (1) the core of the Shapley Supercluster
with radius 13 $h_{50}^{-1}$ Mpc; (2) the core plus A1736; (3) the core, A1736
and the western extension (A3528-A3530-A3532) and A3571; and (4), adding the
northern cluster pair A1631-A1644 to structure (3), the Supercluster as a whole
enclosed within a radius $\sim 90 h_{50}^{-1}$ Mpc. The observed total masses
range between 3.5-8.5 $\times 10^{15} M_{\odot}$. The mass values derived from
the observed intracluster gas (assuming a baryon fraction consistent with
primordial nucleosynthesis) are of the order of few times $10^{16} M_{\odot}$.
Given these estimates, the core is a bound structure with a very significant
overdensity of at least 5 times the critical density, indicating that is
approaching maximum expansion before collapsing. Structure (2) has an
overdensity of 1.7 on a scale of $\sim 30 h^{-1}_{50}$ Mpc. The core is then a
2--3.7 sigma fluctuation in an initial Gaussian perturbation field, normalized
for galaxies and clusters. The highest value applies if we extrapolate to a
density contrast of 200 and assume that $\Omega_b < 0.095$. The baryon fraction
of the core of the Shapley Supercluster, with A1736, is about 15 per cent over
a radius of 28 $h^{-1}_{50}$ Mpc.
