incertae sedis vs. sedes

[Nico Mario Franz]

Hi Taxacomers:

   I'm facing an issue of placing some genera into new subtribes. Can I
just define those subtribes phylogenetically by pointing at two of
their members' DNA bar codes?

Just kidding.

   Actually, I'm wondering when to use the term Incertae SedIS vs. the
plural Incertae SedES? Incertae means something like "unsure" and
sedis/sedes "seat(s)", right? If I placed, say, a single genus in a
subfamily but had no tribal assignment for it, that ought to be a (list
subfamily, then) Incertae sedIS. But what if I did that to 5 genera at
once? Incertae Sedes? When is the plural adequate (if at all)? Does it
(the plural) refer to one unsure place or (potentially) several unsure
placings?

I hope that's a worthwhile question and appreciate any clarification.

Best,

Nico