  Various mechanisms have been developed to explain the origin of Majorana
neutrino masses. One of them is radiative mass generation. Two-loop mass
generation is of particular interest because the masses and couplings of new
particles propagating in the loop can be in the range testable by other
experiments and observations. In order for the radiative mass suppression to be
reliable, it should be guaranteed that lower loop contributions are suppressed.
Based on loop topology and the form of electroweak presentation of the
particles propagating in the loop, one can determine whether a lower---and
therefore dominant---loop contribution is possible. We present a model based on
these general considerations which leads to neutrino masses via a two-loop
diagram which we dub as "snail-diagram". The model has two natural candidates
for dark matter one of them being a neutral Dirac fermion which can satisfy the
conditions of the thermal freeze-out scenario by annihilation to lepton pairs.
We comment on the possibility of explaining the GeV gamma ray excess observed
by Fermi-LAT from the region close to the Galaxy Center. We also discuss
possible signals at the LHC and at experiments searching for lepton flavor
violating rare decays.
