1 The copyright for the schematic backbone drawings in this article (Figs. 1, 14, 52, 53, 62, 71-86, 87b, 88, 89c, 90b and d, 92b and c, 94b, 96b, 102b, 103b, 104b and 105-108) is held by Jane S. Richardson. Upon application to her, she will make these figures available, free of charge, for nonprofit scientific or educational use.
2 Mailing address: 211 Nanaline Duke Bldg., Box 3711. Duke University. Durham. North Carolina 27710.