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I think that I know the answer but just in case...some other opinions are welcome.
The adult snake was found in SW Slovenia where normal coloration of adult viridiflavus is black.
There are some speculations about gemonensis occuring in the area but until now(as far as I know) no proof.

after looking 5 minutes on the picture, I would say a not black viridiflavus
but the white corners on the dorsal scales

We had those colored specimens a few times on Krk, and have never been sure on their idendity (there are no real differences in scalation).

Mario

some more pics...

Mario Schweiger wrote:there are no real differences in scalation

Isn't the number of ventralia supposed to be decisive, Mario?

OK, Jeroen - I was thinking on the scales visible on the pic(tures).
The "Handbook" gives 130 - 180 for gemonensis and 160 - 227 for viridiflavus.

Mario

The snake on the pics was a young adult male longer than 1 meter...also the head shape suggests viridiflavus.
All the gemonensis I saw in Croatia were smaller ...

Since you clearly have caught the snake for the photos it shouldn't be too hard to count the ventrals on the spot, isn't it?
That would answer your question more or less immediately...

Anyway, it's a brownish coloured specimen of H. viridiflavus, which can be found in the SW Slovenia and NE Italy. Like Mario mentioned some time ago, a specimen like this one was probably responsible for description by Laurenti of the animal near Gemona (NE Italy), which is quite far from H. gemonensis distribution. Also, no gemonensis was ever found after that in that area, only viridiflavus.
More or less the same thing with H. gemonensis sightings in Slovenia, almost all of them can be contributed to light-coloured viridiflavus.

I did not realise that the ventralia overlap range, as mentioned by Mario, is that large. That would mean that even after counting all those scales it could still remain unclear, I guess? Pfff...

Something in the headshape and colour of this specimen says viridiflavus to me, but I think that if the pictures would be hidden in a report from e.g. Montenegro and labelled gemonensis, I would never notice. I even once, without knowing the origin, thought that a gemonensis from Crete was a viridiflavus .

Tomaz (or anyone else), how far from the Slovenian border are the closest gemonensis populations located? Is it really unimaginably impossible for them to occur in Slovenia (although it becomes of course increasingly unlikely with more cumulative search effort etc.)?

Is there any chance at all of hybridisation?

Jeroen,

hybridization? I don´t think so!
For example on island Krk they share the same habitats and occur there - on a few spots in high numbers - together.
Only a few times we had specimens, beeing unable to say which of both species they are, although I´m nearly sure, those have been wrong colored viridiflavus.

As far as I know, and from own observations, on Istria west coast, the northern border of gemonensis in the Limski canal - or a few hunded meters more north. We had one specimen on the south facing slopes there, altough just a few hundred meters west of the road, where the snakes easily may go north.

Mario

I didn't say it's impossible for gemonensis to occur in Slovenia, it's very likely, since it's present in croatian Istria. We have a number of sightings of supposed gemonensis, but all but 3 of them are located in the area where the brownish viridiflavus specimens occur.

If gemonensis is present in Slovenia, I would say that it's a bit more to the east, away from the coast but still in a very warm climate, where the other 3 sightings occurred. Further searches will be needed to prove or disprove this, but I believe that gemonensis is absent from the immediate coastal area and the area along the Italian border.

Mario, my book says ventral count for viridiflavus 187-227 (after Schätti & Vanni 1986) and 160-187 for gemonensis (after Henle 1993).