31 out of 33 times when Metoro said 'hônu' he referred to glyphs of the GD17 type.

A few preliminary remarks and imaginations:

1.

"An iconographic study by Jeff Kowalski suggests a cosmological layout for the Nunnery. The higher placement of the North Building, with its 13 exterior doorways (reflecting the 13 layers of heaven), and the celestial serpents surmounting the huts identify it with the celestial sphere. 

The iconography of the West Building, with 7 exterior doorways (7 is the mystic number of the earth's surface), and figures of Pawahtun - the earth god as a turtle - indicate this to be the Middleworld, the place of the sun's descent into the Underworld.

The East Building has mosaic elements reflecting the old war cult of Teotihuacan, where tradition had it that the sun was born; thus, this may also be Middleworld, the place of the rising sun. Finally, the South Building has 9 exterior doorways (the Underworld or Xibalba had 9 layers), and has the lowest placement in the compex; it thus seems to be associated with death and the nether regions." (The Maya)

While we modern people in the western societies probably think of the turtle as the emblem of slow motion, ancient peoples had other key associations. To begin with, the turtle is neither flying high like the birds nor swimming far down in the sea like the fishes. A turtle is living at the surface of the sea or crawling on land, i.e. in the middle world. The sky roof contains air, the bowl of the sea has water and in the middle is the earth we live in. Clearly the carapace of the turtle has an upper shell and a lower shell and in the middle we find the living creature.