Some sort of Oyster mushroom

[Jazzbonick]

Hi there, I live in the French Alps. Exploring the forest I came accross a fallen tree with a great collection of these mushrooms as well as a number of bracket fungi.  They have caps of various colours from brownish changing gradually to blue gray of the big one(two really) in the middle of the big one. They grow at one side from the stalk in the manner of oyster mushrooms. They spring from èèèèèèa rotting tree trunk. I have not found more than any two growing from the same stalk. The gills are decurrent and colored white to yellowish. Information from other sites convinces me that they a type of pleurotus and edible but I would like to know exactly which

[Dave W]

I think these are Sarcomyxa serotina, commonly called "Late Fall Oyster", "Greenbacks", or "Olive oysterling." Some field guides still use the old scientific name, Panellus serotinus. If correct then for at least a few of these the gills should terminate at a short stubbly stem. According to Wikipedia there are actually two species, one that tastes bitter and another that is edible. https://en.wikipedia.org/wiki/Sarcomyxa_serotina

Another possibility is that these are darker than usual Pleurotus mushrooms, maybe P. ostreatus.

I think a closer examination of the undersides would settle any doubt. The gills of Sarcomyxa are not as wide and tend to be more tightly-spaced than for Pleurotus. The gills of Pleurotus will often run part way down a short stalk, but when Sarcomyxa has a stalk the gills terminate where the stalk begins. Also, if you collect a substantial spore print on a non-absorbent surface (white, clear glass, or aluminum foil) then if the print has a yellowish tinge they're Sarcomyxa. Finally, this can be completely settled with a microscope that allows for measurements to be made at 400x magnification (or better). The spores of Pleurotus species are significantly longer and wider than those of Sarcomyxa.