Furycalm Snarl
Land
As this land enters, you may reveal a Mountain or Plains card from your hand. If you don't, this land enters tapped.
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A seething locus of reverent rage.
Illustrated by
Strixhaven: School of Mages#266RareEnglishLegality
Rulings
You may reveal any land card with either or both of the appropriate subtypes. It doesn't have to be a basic land card.
The "Snarl" itself doesn't have any land subtypes. You can't reveal one to satisfy the ability of another.
If a land card with an appropriate subtype is entering the battlefield from your hand at the same time as one of these lands, you may reveal the other land to have the "Snarl" enter untapped.
If an effect instructs you to put one of these lands onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.