  Two members of the iso-structural R$_2$CoGa$_8$ inter-metallic series,
Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$, have been studied by powder neutron
diffraction. Antiferromagnetic ordering of the rare-earth sublattices was
confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination
of the critical exponent showed Er$_2$CoGa$_8$ to adopt a 3D universality
class. In spite of a common magnetic easy axis and similar structural
characteristics, the antiferromagnetic structures were found to be different
for the erbium and thulium based compounds. The corresponding magnetic space
groups were determined to be $P_{2a}mmm'$ and $P_Cmmm$. The difference in
magnetic structures is discussed based on crystal electric field effects that
are known to be prevalent in such materials.
