^O. arenaria^, the sand- or ghost-crab. The name /Ocypoda/ means "swift-footed," and, as it implies, this species is especially noted for its rapidity of movement. These crabs are the opposite of the strong-armed, thick-shelled, slow-moving /Cancroidæ/. An instance is told of a collector having great difficulty to keep up at full run with one which he chased for a considerable distance over the sand. They are also dexterous in burrowing, and live in holes, often three feet deep, dug perpendicularly into the sand. They wander far from their burrows when the tide is out, and every little while raise their stalked eyes and stand on tiptoe to look about. If alarmed, they run with great rapidity to the nearest burrow, or, if danger is close, press themselves on the sand until an attempt is made to touch them, when they again dart away rapidly, and in running hold their bodies high, and double and dodge so that it is difficult to catch them. /Ocypoda/ is colored almost exactly like the sand, and this mimicry, together with its fleetness, makes it interesting to note and difficult to capture. It inhabits sandy beaches above tide-mark from Long Island to Brazil, and subsists largely upon the beach-fleas, which inhabit the same localities. It springs upon them, very much as a cat catches a mouse. The carapace of this species is almost square in outline, and on the anterior corners ends in a spine. A small portion of the carapace folds down like a band between the eyes. On each side of this band, and extending across the front, are large grooves for the eye-stalks. The body is about an inch thick; the first joints of the chelæ are toothed; one chela is a little longer than the other, and both are coarsely granulated. The other four pairs of legs are thickly fringed with hairs. (Plate LXIII.) [Illustration: PLATE LXIII. Hemigrapsus nudus. Hyas araneus. Ocypoda arenaria. Pugettia gracilis. Libinia dubia. Pitho aculeata.] [Illustration: /Uca/ (/Gelasimus/) /pugilator/, the fiddler-crab; male.] GENUS ^Uca^