Beavers have 6 of these on their heads (in 2 sets of 3); 1 is transparent & helps in swimming underwater
[DOC] [TLE] Managing Wildlife Damage: Beavers (Castor canadensis ... [PAR] Beavers have a compact head with strong jaws and sharp chisel-like front teeth ... 
by their round, dome-like appearance, and may extend 3 to 6 feet above the ... an 
underwater opening, which helps prevent predators from entering the lodge.