THE extinct Order Pelycosauria contained several genera of reptiles characterized by extreme elongation of the neural spines of the vertebrae, which in life supported an area of membrane forming a ``sail''1-3. Dimetrodon grandis was the end form of an evolutionary series of pelycosaurs that had tended to develop increasingly large sails2. Many suggestions have been made about the function of the sail, as camouflage among reeds while it waited for prey, for sexual display, or literally as a sail while swimming2. Romer and Price2 first suggested that it served a mechanical function and strengthened the backbone, but Romer3 later realized that the sail had evolved with features strongly suggesting an early attempt at temperature regulation. The spines are grooved anteriorly and posteriorly to house blood vessels carrying a rich supply of blood to the sail3-5.