  We analyze the $2-10$~keV X-ray emission of complete samples of AGN and
galaxies selected at 12$\mu$m and recently compiled by Rush, Malkan \&
Spinoglio (1993). As a necessary complement we also study the IR emissivity of
the largest sample of hard X--ray selected AGN (Grossan 1992). Our purposes are
to probe the unified scheme of active nuclei and to evaluate the local X--ray
volume emissivity of low luminosity and `hidden' AGN. We confirm that X-ray
emission is basically restricted to AGN, Seyfert 1's being much more powerful
emitters than Seyfert 2's. We find no signal of X-ray emission for the
remaining galaxies selected at $12\mu$m, not even for those having a flat
far-infrared spectrum, which would be prime candidates to host hidden AGN. Our
result on the local volume emissivity of the Seyfert 1's is fully consistent
with those derived from studies of the hard X--ray luminosity function. However
the 95$\%$ upper limits to the local emissivity we derive for the Seyfert 2
class and for the most likely `hidden' AGN candidates, severely constrain (and
in some cases exclude) models of the X--ray background based on the unified
Seyfert scheme. We also find that less than $\sim20$\% of Seyfert 1-like AGN
and less than $\sim 50$\% of Seyfert 2-like AGN can be `hidden' in the
non-Seyfert galaxy samples assuming that the X-ray to 12$\mu$m flux ratios
measured in the 12$\mu$m selected samples can be considered typical for these
classes of objects. In this case, the number ratio of Seyfert 2 to Seyfert 1
galaxies ranges from 1 to 2.
