You cannot have wandered among the rocks on our southern or western coasts, when the tide is out, without having observed that their whole surface, up to a certain level (often very precisely defined), is roughened with an innumerable multiude of little brownish cones. If you have ever thought it worth while to examine them with more care, you have seen that, crowded as they are, so thickly that frequently they crush each other out of their proper form and proportions, they are all constructed on the same model.
