At last...you're here. The ancient civilisation that is Lemuria is now a few steps away. Follow the waterways to an empty dock, and park your boat in there. As Felix, don't get off the ship - instead go inside through the door. Go down the steps to the right and through the right-hand door. Use Parch and climb down to reach a Mist Potion. Now return outside, and disembark from your boat.
Follow the long path around to the statue and use Move. The guards will notice you, and after a random talk you'll get to enter Lemuria. Now, there's a couple of things that you might want to do before you enter any of the houses.
Firstly, there are patches of flowers with butterflies fluttering around them. Some of these hide goodies that can be dug up by Scoop. The one to the left of the huge palace at the top of the town has some Star Dust. The one by the dog has a Bone, which is completely useless and should be dropped, the one to the bottom right of the Fountain has a Lucky Medal, and the one outside Lunpa's house (near the vine you can use Growth on) has a Rusty Sword. Use Growth on the vine next to it (trade 1 Venus for 1 Mars Djinni) and use Cyclone on the weeds at the bottom, past the cow, to get a Hard Nut.
Secondly, the spring at the centre of town has two purposes - drink from the side of it and your character's HP and PP will be fully replenished. Then, talk to the man standing in front of it. You can now play a mini-game whereby you can get lots of good equipment with Lucky Medals. But make sure that you get at least one Lucky Medal in the centre - the first time you do this nets you a combo tablet, Eclipse! Eclipse requires 2 Mercury and 3 Jupiter Djinni to summon, and may lower enemy Defence. Anyway, keep throwing Medals in to get items, then re-configure your equipment to bolster your characters up. The closer you get your Lucky Medal to the centre, the better the equipment is, although you may want to purposefully miss the centre by bouncing it off the animals so you can get some other stuff.
When you're ready enter the house to the left of the stairs to the palace. Talk to the man. It's actually Piers' uncle. When you regain control of Felix (after Piers leaves you temporarily), go to the tall house on the right-hand side of town, where the Growth vine is. Lunpa will remark on the door being broken, so you need to use Lash to climb in through the window. If you do not have Lash (because the Lash Pebble is on Piers), then Lunpa will drop a rope down to you, so don't worry about who has the Lash Pebble.
Go down the stairs and approach Lunpa. After the discussion, you can enter the palace. Inside, you will have a discussion about the decay of Weyard with Lord Hydros, then you can bag the Grindstone from the chest that Lunpa gives to you. You can use this to get to the Western Sea. Now, leave Lemuria and get back on your ship. After the amusing scene, head back into Lemuria and use Grind on the large rock blocking the path to the left. Go around to the next dock.
You might be expecting this to be a very long and very important dungeon, but thankfully it isn't. There's nothing under any of the weeds except the one on its own on the other side of the archways that you can't get past. Use Cyclone within range of it and you'll uncover a Mercury Djinni, who hides in the statue. Use Tremor to remove it, and then talk to Rime to have it join your party (it should be on Piers).
Leave Ancient Lemuria and go back to your boat. Go back through the waterways and sail back into the Sea of Time. In the area where you fought Poseidon, use Grind on the rocks to the left and you can sail directly out of the Sea of Time that way. Before you go anywhere else, sail back to Yallam and forge whatever you collected. Sell off any extra equipment you don't need anymore.
You remember Gondowan Cliffs, right? You can now go through there in your ship - the rock that's there can be eroded with Grind to allow access to the Western Sea.