If you’ve not seen the mystery presented, check out the original posting first
As I mentioned in the previous post, it was only a bit of a coincidence that gave me a plausible idea of what these rubber-egg things were. As you may have already noticed if you scrolled down to see the pictures, in addition to the three that were present on the first post, there’s also one of an earthworm. This earthworm was also under the piece of plastic, but it did not seem to be particularly associated with the mystery eggs. I just grabbed a shot because I already had my camera out and I don’t see earthworms in the forest very often (and I don’t think I’ve ever taken pictures of them before). It turned out to be good that I did.
When I was working on tagging the photos from the day, I looked up earthworms to see how they are classified, so I could put the tag in the appropriate place. As a result, I got to reading the wikipedia page for earthworm and happened to notice a picture on that was captioned “earthworm coccoon”. I had never heard of such a thing, but what I saw in the photo looked pretty similar to what Rowan had found in the forest, so unless/until I hear a better idea, it’s my best guess at the answer to this mystery.
I wrote most of this at the same time I wrote the original post, so at the time had seen no guesses (besides my own). AT this time (a week later) both guesses (as of this posting) were for banana slug eggs. I had not actually considered that as a possibility, in part because the previous slug eggs I’ve seen had been much smaller and there were a cluster of them. Of course those were not eggs of a banana slug (Ariolimax columbianus) but instead one of the smaller slugs that is abundant in yards/gardens. In hindsight, an obvious guess would have been banana slug, as they presumably would have larger eggs. That said, in a brief search on-line, I did not find any pictures that would lead me to conclude one way or the other about whether these were a good match for banana slug eggs. I suppose in the end, the best way to learn would have been to collect them and see what came out. I need to get back that way soon, so perhaps I’ll take side trip and grab them (if they’re still there) to see if they will hatch.