This is a raccon vent trap with a one way door. It goes into a single chamber trap. Note: the raccoon can come into the trap as it leaves its denning site in the chimney site, but cannot regain entrance.
Click image to enlarge
Notice that this chimney traps is set up on a round chimney flue. The flue on the left is not functional. The one in the middle is being tested for raccoon activity. The flue on the right is a round flue. The contractor has cut a piece of plywood with a hole in it to stabalize the trap and allow the raccoon vent into the trap.
Click image to enlarge
Click image to enlarge