So what do you call the name Larix xmarschlinsii?
The name of a notho- (i.e. hybrid) taxon, more specificially of a hybrid
arising spontaneously in cultivation.

Iris Cohen schreef in berichtnieuws
So it is the same as xFatshedera lizei.
Now where do we put xCitrofortunella mitis (Calamondin)? Is there some
location where the natural habitats of tangerines & kumquats overlap, or did
it arise in cultivation in the distant past?

One of the interesting aspects of hybridizing, at least in orchids, is
that occasionally there is a registered artificial hybrid which is
subsequently discovered to be also a natural hybrid. The latter is published
by a botanical taxonomist, following which the poor plant is doomed forever
to carry two different names. According to RHS rules. If it is used as a
parent in hybridizing, it is called by the registered name. If the natural
hybrid discovery comes first, the plant only has one name, but it may be
written differently or carry a Latin ending.
Iris,

+ + +
Orchids have their own set of rules, but neither the ICBN nor the ICNCP, as
they stand, makes a distiction between natural and artificial hybrids. There
are just hybrids.
PvR