Ptiliogonys is formed from πτιλον,
ptilon, feather (with an unexplained added
i after the
l) + γόνυ,
gonu / gony, knee +
an added -s.
In principle, the modified ending precludes using the stem of the original final element of the type-genus name, which is how "Ptiliogonatidae" was formed (the genitive of γόνυ is γόνατος,
gonatos, and the stem is
gonat-); the stem should be formed assuming a latinization with a change of ending, and be "that appropriate to the latinized form" (
ICZN 29.3.2).
Latin words in -ys (quite unsurprisingly, these are all of Greek origin) have genitive in -yos/-yis or (less frequently, feminine words only) -ydis, hence Ptiliogonyidae or Ptiliogonydidae would probably be more correct than any of the two forms that are in use.
On the other hand, any of the two forms that are in use would be protected by
ICZN 29.5 if it was "in prevailing usage".