Hrdlicka, Aleš. 1904. "Anomalous articulation and fusion of the atlas with the occipital bone. Abstract." Washington Medical Annals, 3, (1) 1–2.
From time to time I pose this question to medical students and residents. It's an unfair question; indeed, it's something of a riddle, as it requires a read-my-mind approach to the correct answer. But ...