While I don't know any larches personally, I occasionally have to label
them for a bonsai show, so this has me puzzled. The usually accepted name
for Dunkeld larch is Larix xeurolepis, which dates from 1919. Larix
xmarschlinsii, dating from 1917, is on the RHS checklist of accepted names
of conifer species (for registration of cultivars). However, IPNI only lists
L. xeurolepis, opening the question of whether xmarschlinsii was validly
published. And W3TROPICOS lists neither of them, although it lists several
other larch hybrids.
Can anyone throw further light on this?

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Some notes:
- the RHS list accepts Larix xmarschlinsii
- so does Mabberley, starting in 1987
- W3TROPICOS list four hybrids, but at least three of them are 'natural
hybrids' occurring in the wild. Maybe the fourth is one too. And Tropicos
may list only 'wild taxa'???
- The fact that IPNI does not list Larix xmarschlinsii does not mean
anything except that it does not list it. Listing by IPNI is not a
requirement for valid publication (it was proposed that all new names are
required to be submitted to a registering society, which would certainly
make life easier for anybody looking for a name. It was sunk without a trace
in 1999 at StLouis). Although IPNI is the best we have got, there are quite
a few names not lited by IPNI.

I am still going for the 1980 publication. Of course if this 1980
publication were to happen today a proposal to conserve Larix xeurolepis
would be the thing to do. In 1980 this was not yet possible.
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A quick Web search shows that Dunkeld larch still goes by both names. A
history of the tree is a bit cloudy. The Japanese larch was introduced to
the Atholl Estates, & the Dunkeld larch subsequently developed & was planted
there. So is it a true natural hybrid altogether?
Iris,

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Since the two species do not naturally meet it is not a natural hybrid!
PvR