1. Russell's Second Conjecture, that "It's turtles all the way down", has only been demonstrated for the first 107 turtles. It has been shown to be mathematically equivalent to the Riemann Hypothesis, so if a tortoise or terrapin is found, the corollary follows that there must be a corresponding non-trivial zero of the Zeta Function with a real part ≠ ½.
Both conjectures are equivalent to Dunsany's Recursive Theology, which states that it's chess-playing gods all the way up.
2. It is estimated that at least 750 softball games around the world are disrupted each year by leatherback turtles.
3. Turtles are clockwork entities, devised by James Blaylock as part of the backstory of The Digging Leviathan, only to be released into the environment when anti-steampunk extremists broke into his laboratory.
4. Hoan Kiem Lake in in the middle of Hanoi contains a giant softback turtle of uncertain species, recently captured with the help of the Vietnamese military who are better-trained in chelonian wrangling than any other military organisation of comparable size. It may be the last of its species. The mummified turtle kept in a shrine on Jade Island (after dying in a crowbar-related episode in 1967) was probably its mate. One day REAL SOON NOW the one in the lake will notice its bereavement and wreak a bloodthirsty vengeance. Someone should write a book.
5. Speaking of books, Mock Turtle sounds interesting and I have asked the Library Pixies to interloan a copy from the Pseudobiblic Archives.
axolotls with their Aztec profiles, or for alien species, but it's not so much fun when you're a stomatopod.
a Russell Hoban novel. Not that there's anything wrong with that (especially since Russell Hoban himself was only a character in a Dorothy Sayers novel).