> The gen.pl. of <viridis> is slightly unusual -- <viridum>, not
*<viridium>.
Only once, in Poetry, where the form viridium will not scan. Elsewhere we
get viridium as normal (and viridia for the neut plural). So that's not a
problem for us.
The -is ending rather than -us looks like analogy, and Piotr's suggestion of
virilis as the source seems excellent.
Peter