R' Shimon says: There are three levels of bikkurim —- the [primary] bikkurim, the supplement to the bikkurim, and the “crown” of the bikkurim.  The supplement to the bikkurim are [fruit] of the same kind, while the “crown” of the bikkurim are [fruit] of a different kind.  The supplement to the bikkurim is eaten in taharah and is exempt from demai; the “crown” of the bikkurim is liable to demai.