There are differing opinions on whether T. myomyces is a good species or a synonym of T. terreum. In my opinion they are synonyms (following Fungi Northern Europe volume 4). Current sequence data suggests the use of the name T. myomyces in North America is usually a misapplication of T. triste or T. bonii but there isn't much on this in the literarture.

So, I would support synonymisation but I'm not sure much can be done with North American misapplications

I think unless we can find something in the reviewed literature to confirm North American misapplications then they should also be treated like European records. I can see the misuse from deposited sequences but I think we need more than that as justification.

@jameskm could you do the split please. I'm not confident with atlasing.

Make the parent Tricholoma terreum

No. You need to do it manually first.
If you tick 'move children to output' then as far as I recall it would automatically create the invalid Tricholoma terreum var. cystidotum. We should not be inventing and propagating invalid names in iNat. The current design inappropriately assumes ICZN rules. I have had this discussion in the past and staff need to wake up to the fact that ICN is not the same as ICZN and the current design provides no other option. Better taxonomic/nomenclatural design (and the observation identification design) would allow that, but we stuck with current system. Please do not invent invalid combinations.

ok - good.