Extreme reduction of dermal plates on the dorsal surface of body, with just the ventral and mid-ventral rows normally developed; absence of dorsal-fin spinelet and nuchal plate; absence of the connecting bone and ligament between second dorsal-fin proximal radial and rib of sixth centrum; reduction of lateral line; loss of the infraorbital sensory canal; and absence of adipose fin (versus present in all species but H. vestigipinnis). Fourth branched ray of pelvic fin longest, reaching to middle of anal fin when depressed.