I don't know much about how primitive humans made holes in shells, but the radula of a gastropod produces a very characteristic hole, larger on the outer surface of the shell than on the inner surface, with sloping or slightly concave sides. The gastropod make a concave or "bowl-shaped" depression in the shell it is attacking, then continues to enlarge and deepen that depression until the bottom of the "bowl" breaks through the inner surface of the shell. Under magnification (microscope), the sloping sides of the depression show the grooves or scratches from the radular teeth of the gastropod, and these scratches generally run more or less vertically up and down the slope of the hole. As I said, I don't know what kinds of tools people might have used, but I would assume it was either a smooth, sharp tool like an awl, or some kind of rotary rasp. I doubt that these tools would produce the fairly regular and uniform impressions that radular teeth produce, and if a rotary tool did produce similar marks, I would guess that the scratches might run more or less horizontally, around the hole rather than up and down its sides. The fact that most of your shells are drilled near the umbo is probably not helpful. Many carnivorous gastropods prefer to attack that part of a bivalve shell, and I have seen beaches with many dead, "drilled" shells, virtually every one of them drilled near the umbos. (I often wondered why this is so, since that tends to be the thickest part of the bivalve shell. Any ideas on that, anyone?) If humans drilled holes in shells in order to use them as adornment, either hung on a cord around the neck, or stitched onto clothing, or laced onto other artifacts, then it seems the umbo area might be the most likely place for them to drill as well(?) As for octopus holes, I have less experience with them, but my impression is that they are more like a puncture, and might be difficult to distinguish from a hole made by a small awl(?) Hopefully someone with direct experience can address this. Regard Paul M.