Why does kino open the oyster last

Kino was afraid his eyes had deceived him, thus, he saved the "great oyster" for last.

Very deliberately Kino opened his short strong knife. He looked speculatively at the basket. Perhaps it would be better to open the oyster last. He took a small oyster from the basket, cut the muscle, searched the folds of flesh, and threw it in the water. Then he seemed to see the great oyster for the first time. He squatted in the bottom of the canoe, picked up the shell and examined it. The flutes were shining black to brown, and only a few small barnacles adhered to the shell. Now Kino was reluctant to open it. What he had seen, he knew, might be a reflection, a piece of flat shell accidently drifted in or a complete illusion.