the number of species diminishes in the ratio of distance. About two hundred species exist in collections, of which about ten are found on the British coasts. The shells of this genus are generally remarkable for elegance of form, and often for brilliancy of colouring. Our British species are among the handsomest of our bivalves. "There is a pleasure," observe the learned authors of the British Mollusca, "in investigating such a group as this, wherein we find not only the greatest variety, but also the greatest distinctness and consequent certainty of specific determination."