Good lord.
Live worms, dead squid, and dead frogs here (the latter were freeze-dried. It was a heck of a lot of fun trying to cut open the carcass of a freeze-dried animals with blunt scalpels).
@The Mountain raises a good point; the types of animals dissected across the country could vary, but the thing of it is ultimately this: Jon autistically curated a cat skull and fur, and then used them as decorations.
Let's put that in perspective, shall we?
Did Jon replace the window that his brother broke? No.
Did he do anything to stop the flow of rain water from damaging his wall? No.
Did Jon fix his wall when the rain inevitably damaged it? No.
Did Jon learn new ways to save files from his computer that didn't involve using a dying, antiquated desktop? Not until getting a new laptop, which he couldn't have bought for himself.
Did Jon take care to maintain, as best as his addled brain could manage, a cat skull from his high school years?
Yes.
Once again, we see where his priorities lie. And, as if there's some cosmic law that demands it, those priorities must be unswervingly disturbing.