The precise appearance of the "white apes" is never clearly defined. Based on the context of the story Facts Concerning the Late Arthur Jermyn and His Family we know that the remains of that they are habitually bipedal, have a roughly similar frame as anatomically modern humans, but would be recognized by elongated arms, an unusually robust strength and an inhumanly formed facial structure. We know that they are at least biologically and genetically compatible on some level with human beings, as Arthur Jermyn is revealed to be the descendant of a "white ape" female and a human male.