You can do Boolean searching, just type the AND, OR, and NOT in capital letters in the search box. You can also use the minus sign instead of NOT, eg, extinction ("prehistoric animal" OR dinosaur") -australasia -australia -oceania.
This will find articles about extinction of either dinosaurs or prehistoric animals, but not with australasia, australia, or oceania in them. Go to the CrossSearch help page for more tips about this
You can always use the limits in the column to the left of your search results to refine the search further.