"These range from 2,6-dimethylheptanoic to 5,9,13,17-tetramethyloctadecanoic acids, but those encountered most often are 3,7,11,15-tetramethylhexadecanoic (phytanic) and 2,6,10,14-tetramethylpentadecanoic (pristanic) acids. 4,8,12-Trimethyltridecanoic acid is especially common in fish and other marine organisms." . . . .