Coiled snake with open jaws

There was a scribe there called Brother Aldous, who had the steadiest hand in the whole fen country and the worst temper before Sext. He copied psalms by a window that looked out over the reeds, and one February morning he found a thing coiled on the sill where the rain never reached. It was black as a wet rope, banded like a barrel, and its head was lifted above the lintel with its jaws wide open, a red mouth as bright as a cardinal's hood. Aldous had never seen a serpent in his life. He took it for a devil and nearly fainted into his ink.

Now, the old abbot said the snake had come up out of the marsh to see what the monks were doing with their days. Brother Aldous said it was hungry. Whichever was true, it did not bite him. It only leaned out over the desk and let fall three dark drops from its jaws, one, two, three, onto the unlined vellum he had been saving for the great Psalter. Where they landed the skin drank them up and went glossy as a hazelnut.

He wanted to scrape it. He had the knife in his hand. But the snake watched him with a gold and blue eye, and he thought, very suddenly, of his mother reading him the names of the saints. So he dipped his pen in the black where the drops had fallen and wrote the first word of the psalm in letters so tall they climbed the margin. The snake coiled tighter, pleased, and was gone by Compline.

Every page of that Psalter begins with a drop of something that is not quite ink. Hold the lamp to the first letter and you will see a little wet red shine in the corner. That is the mouth. That is how you know the book is finished.