"The turtle altar on the west side of Stela C balances an uncarved rectangular altar on the east. The turtle altar has two heads that match the images of the Cosmic Turtle as it was represented on pottery. Glyphic texts elsewhere make clear that this turtle was ak and the cleft in its carapace was an ol, or "portal." He is the Cosmic Turtle that floated in the Primordial Sea and sat in Orion."
Linda Schele & Peter Mathews, The Code of Kings: The Language of Seven Sacred Maya Temples and Tombs, p. 145-6