Kingdom: Fungi
Division: Basidiomycota
Class: Agaricomycetes
Order: Agaricales
Family: Agaricaceae
Genus: Agaricus
Species: A. xanthodermus
Binomial name: Agaricus xanthodermus
Synonyms: Pratella xanthoderma, Psalliota xanthoderma, Fungus xanthodermus
Common name: Yellowing Mushroom, Yellow-staining mushroom, Yellow stainer

Agaricus xanthodermus is a saprobic species of mushroom of the genus Agaricus. It is poisonous for most people, causing severe gastrointestinal upset and gives rise to symptoms of sweating, flushing, and severe stomach cramps but it can be eaten by some people without apparent negative effect.

Agaricus xanthodermus has a 15 cm diameter white cap that bruises to yellow. The cap has a similar shape to that of the edible field mushroom.
The gills are whitish to pale pink, becoming brownish with age.
The bulbous stem has a large, prominent, floppy ring. 
This mushroom has an unpleasant, characteristic carbolic smell. The smell is especially strong at the base of the stem.
It grows in hedgerows, woods, paddocks, lawns and gardens. It occurs during summer to autumn.
Agaricus xanthoderma can be easily distinguished because it displays a strong yellow colouration at the base of the stem when cut.

1-Agaricus xanthodermus Genev 181971.jpg 

1-Agaricus xanthodermus Genev .jpg

The strong yellow colouration at the base of the stem when cut.1-Agaricus xanthodermus section.jpg

1-Yellow stainer mushroom.jpg

Thanks to Wikipedia for text and information: