Dr. McCoy, aka Bones, has a couple of weird skulls in his office, as seen in the following picture from the episode "Is There in Truth No Beauty?" (they are highlighted with red arrows and the frame is from around the ~47:21 mark):
They definitely look non-human and are probably not from any other Federation species we have seen in the series, so my question is: which species are the skulls from?
In-universe answers are preferable, but out-of-universe trivia about the props might be considered as well.