Definition of "sixgill"

“Shark After Dark” features scientists swimming with their subjects as they investigate their nocturnal feeding habits; residents of the Seattle area will be interested to know that well inside Puget Sound, quite large sixgill sharks come to the surface for dinner within 150 feet of shore.

2009 August 1, Mike Hale, “Ah, That Jersey Shore: The Fish Are Really Biting”, in New York Times‎