Maturin is one of the guardians of the Beam that support the Dark Tower. The Turtle is said to have vomited out our Universe when it had a stomach ache (and IT thought that Maturin died after this). He bears a resemblance to the World Turtle of Hindu myth. He also has a grandfather-like voice and is shown to be kind and loving.
Roland recalls his memory of the twelve guardians of the beam, including a poem we've heard before, in It.
THE GUNSLINGER RESUMED: “At each of the twelve lesser portals the Great Old Ones set a Guardian. In my childhood I could have named them all in the rimes my nursemaid-and Hax the cook-taught to me ... but my childhood was long ago. There was the Bear, of course, and the Fish ... the Lion ... the Bat. And the Turtle—he was an important one ...”
The gunslinger looked up into the starry sky, his brow creased in deep thought. Then an amazingly sunny smile broke across his features and he recited:
“See the TURTLE of enormous girth!
On his shell he holds the earth...
- Book 1 / Part I / Chapter 13