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High magnification polarized light image of a Paranthropus boisei molar showing cross-striations. These light and dark bands run horizontally across the vertical enamel prisms, and are spaced approximately 5.5 microns apart. Fifty to 55 lines can be counted from the bottom to the top of the image, representing 50 - 55 days of tooth growth.
Image credit: Tanya Smith (Harvard University)