Unseen Menagerie

As Unseen Menagerie enters the library zone,
choose a creature type. Reveal the top 10 cards
of your library and put all creature cards of the
chosen type face up on top of this card as a new
library. When this is empty, put it into its
owner's graveyard.
Whenever you cast a creature of the chosen type,
you may return it to your hand then put a
creature of the same converted mana cost from
this library onto the battlefield.