Well, I can say that posting screenshots of the structures isn't likely to help you any, or at least won't help you to find them, only to recognize them when and if you do locate one of them. I've personally encountered both of the underground structures, and in fact my current base is located pretty much right next to the 'lesser' of the two. Having said that, the Lexicon does give you a hint on how to find the lesser of the two; it mentions lights at night to be a bit more specific. The other structure I've encountered... well, I encountered it back on my test world (now quite gone due to significant world generation changes in my present pack) and haven't located it yet in the current world. For what it's worth Reika, you had me a bit scared when I encountered that one. Again, the Lexicon does mention this structure, though doesn't really give much by way of hints beyond 'It's underground, pay attention!' to summarize.
On a semi-related note, is the 'lesser' structure supposed to regenerate, Reika? The one near my base has magically reconstructed itself at least once since I started the world, and I've been checking back on it every couple of in-game days to see if it does again (to no avail thus far). While I don't particularly need new knowledge fragments at this time, it'd be a handy method of replenishing them since I've had absolutely no luck in finding another of those structures in my explorations, which should be relatively easy with flight. Alas, I would be mistaken.