Warning: Myst and Riven spoilers ahead.
As I was studying for my final last night and this morning, a puzzle idea occurred to me. What I've been wanting to do is to make the player examine the flora and fauna around him carefully instead of just looking and thinking, "Pretty," and then immediately leaving when all the sightseeing is done. There's no better way to force a player to stick around then with a puzzle, so here's the plan: the Writer of this Age has a lock with a combination, say 3, 5, 2, 6, 5. To help him memorize the number, he invents a mnemonic based on fundamental numerical animal and plant characteristics. For example, lepidodendrons branch bichotomously, each branch splitting in two. That two-splitting is a fundamental characteristic of the lepidodendron, so it would be represented by the value "2." Blastoids and crinoids have pentameral radial symmetry, so they would be represented by a "5." (Gehn's favorite!) Some flowers have hexagonal symmetry, so they would have the value "6." Trilobites are named tri-lobe-ites because they have three lobes, both from back to front and from left to right. Sort of like how insects have a head, thorax, and abdomen, only trilobite style. So anyway, the mnemonic would be something like, "trilcrilepfloblas."
Presentation and the withholding of information are the keys to this puzzle. Firstly, we want to be subtle about giving hints, and we want the presentation to seem natural. My plan is to have the guy mention in a journal entry that he got tired of going back to Relto to look up the combination to the lock since he kept on forgetting it, and so he memorized a mnemonic to help him recall the number. Immediately the player's ears ought to perk up and he should be thinking, "If I can just figure out what that mnemonic is, then I should be able to open that lock! I must find the mnemonic!" But I'm not going to write it in the journal for them to conveniently read--I mean, who would write their combo mnemonic in the journal they left lying plainly on their desk in an unlocked room? (Actually, I'm pretty sure Chacal could think of some people who would. But anyway.) Nosireebob, the character wrote his mnemonic twenty or so times, then crumpled the sheet of paper up and threw it into his trash can. But you can remove it, uncrumple it, and read it if you click in the trash can at the right spot. Upon seeing the word "trilcrilepfloblas" written over and over, the player should guess that this is the mnemonic. Now all he has to do is figure out how to decipher it. I'm sure if he ponders it over long enough he'll eventually pick out the abbreviated organism names.
Now here's where the cruelty begins! The lepidodendrons and trilobites will be fairly obvious to figure out. The flowers will both be subtly connected with a name somehow. Maybe I'll have a journal entry like "So and so left me a beautiful bouquet of Flowername." And then I'll have a tiny pressed flower lying in the margin by the spine so that they can observe the symmetry and figure out the fundamental number. But the reaaally evil part has to do with the crinoids. As I mentioned over in the thread about Marine Carboniferous Life, there is this one species of crinoid that looks like a chicken's foot, only weirdly branched. Well, I'll make a drawing of a chicken-like animal (obviously alien) with crinoids for its feet! (Excuse me for a moment while I laugh in maniacal glee) ...pant, pant...hehe...Okay, then I'll put a clear and unmistakable label underneath the chicken-creature: "Crinoid." HAHAHA! Of course, it's a totally false artist's reconstruction--and I'll have to give the player a hint that it is false by mentioning (probably in the journal) that whoever did the set of reconstructions was somewhat off, amusingly so. (There will have to be more reconstructions so that the fake crinoid can hide amongst them, and I'll have to add one obviously wrong reconstruction--perhaps a t-rex with its arms on its head as antennae--so that the player will notice the obviously wrong drawing and think that they've accounted for the error mentioned in the journal. And maybe they'll even be so distracted by the wrong one that they'll turn it into a puzzle--bonus!) As for an explanation as to why the person who did the reconstructions got them wrong, I figure the character's young son did the drawings, since his father is teaching him paleontology skills. That part is liquid for now though. At any rate, they should be able to figure out the lepidodendron, the trilobite, and the flower easily enough. They'll have the crinoid wrong if they guess "2" for bilateral symmetry (that's what kind of symmetry humans, chickens, fish, and anything else that can be split down the middle to make a mirror image all have), and the only information I'll give about blastoids is that they evolved from crinoids and look pretty much like them. In order to figure out what's up with the crinoid, they'll have to observe crinoids in the wild and connect them with the chicken-creature's feet.
Once they've figured the crinoids out and they're sure that they have the number "5" right, I'm certain they'll try the same number for the blastoids and then the lock will open at last. But they'll have two parts of the code unsolved unless they solve the part about the crinoids, adding an extra degree of uncertainty.
The only thing bugging me now is the lock. I've given them 3 out of 5 pieces of the puzzle, and I don't want them to be able to do a process of elimination solution where they try every possible combination until they have the right one. So I'm either going to have to make it massively inconvenient and time-consuming to enter a combination so that if they try process of elimination it'll take them an eternity to finish, or, I'm going to have to have many numbers possible for each entry instead of just 0-9. For example, in the rebel lock in Riven they had 1-25 to choose from in that animal puzzle, plus those stones took foooreeever to go up and down. It was much harder than the clock tower in original Myst, in which I simply tried every possible time until I came to the correct one. I think I'll have both serious inconvenience plus D'ni numbers that go up to 25, which should work since all D'ni numbers take up the same square space.
Well, this puzzle ought to keep them stuck in the Carboniferous for about an hour. But with any luck it could stall them for three!