You may exile two green cards from your hand rather than pay Allosaurus Rider's mana cost. Allosaurus Rider's power and toughness are each equal to 1 plus the number of lands you control.
When Elvish Harbinger enters the battlefield, you may search your library for an Elf card, reveal it, then shuffle your library and put that card on top of it. {T}: Add one mana of any color to your mana pool.
