As this land enters, you may reveal a Plains or Island card from your hand. If you don't, this land enters tapped.
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There at Pelargir lay the main fleet of Umbar, fifty great ships and smaller vessels beyond count.
(08/04/2016)
You may reveal any land card with either or both of the appropriate subtypes. It doesn't have to be a basic land. For example, you could reveal Canopy Vista from the Battle for Zendikar set to satisfy the ability of Port Town.
(08/04/2016)
Lands don't have a subtype just because they can produce mana of the corresponding color. Port Town itself is neither a Plains nor an Island, even though it produces white and blue mana, so you can't reveal one to satisfy the ability of another.
(08/04/2016)
If an effect instructs you to put Port Town onto the battlefield tapped, it will still enter the battlefield tapped even if you reveal a land card from your hand.
(08/04/2016)
If a Plains or Island is entering the battlefield from your hand at the same time as Port Town, you may reveal the other land to have Port Town enter untapped.
