1. 1 Cedar Lodge, Maenygroes, Ceinewydd, Ceredigion, Wales, SA45 9RL, U.K.
2. 2 Petroleum Geoscience Programme, Department of Chemical and Process Engineering, The University of the West Indies, St. Augustine, Trinidad and Tobago
3. 3 Smithsonian Institution Mathematics and Statistics NMNH MRC-121, Washington, D.C., U.S.A.