The Southern Triangle (Or Triangulum A.) Coin can be found in the Cortica River Upstream in the Twilight Temple at co-ordinates C-2. Its shine spot is in the eastern section of C-2, between the four broken pillars in the North-East quadrant of the box. This coin's zoom-mode glow also contains two blue Guppies. This coin has a quirk, however; it may appear and disappear seemingly at random. If you can't find it try diving again, maybe this time with Oceanne/Oceana, who can see "Shine Spots" better.