A few things,
It looks as though P. atromarginatus has a radially fibrillose pileus whereas P. similis has more of a velutinous or granular-punctate surface, the pileipellis structure is probably quite different.
In P. similis the base of the stipe is covered in squamulose fibrils or flaky scales and is distinctly bulbous, it looks as though P. atromarginatus lacks any scales at the base of the stipe and is not distinctly bulbous just slightly enlarged.
The etymology of the species name similis refers to it’s similarity to other Pluteus species.
E. Horak compares this species with P. spegazzinianus which is known from southern Chile and Argentina and says they share several macroscopic and microscopic characters!
I don’t have access to the published description for P. atromarginatus, if anyone could link me to this I will do a more thorough comparison.
