More or less cylindrical, almost naked; the valves of the shell very small, and almost entirely concealed under the skin; tufts hairy or silky between the parts of the valves near the margin. 3. Chiton. Six species. This genus was so called from the resemblance of its testaceous covering to a coat of mail. The form of the Chiton is very similar to a well-known insect called the woodlouse, found in decayed timber; it generally adheres to rocks, or lies rolled up like a ball among seaweed and stones. In length it seldom exceeds an inch, except in tropical climates, where they are sometimes three or four inches long. In general there are eight valves, the termination of which is surrounded by a scaly or rough ligament, which enables the animal to expand or contract its shell freely. It presents a great variety of colour; in general it is dark brown, overcast with a shade of green, but some are beautifully variegated with pink, yellow, blue, or red; interior green or whitish. Shell more or less elongated, consisting of a longitudinal series of eight very symmetrical calcareous pieces, more or less curved, and round at both extremities; summit more or less marked, and when imbricated, always from front to rear. Chiton gigas. C. squamosus. C. Peruvianus. C. spinosus. C. fascicularis. C. marginatus. _C. squamosus._ The scaly Chiton. Pl. 1, fig. 1. Depressed, valves large, carinated, well imbricated; the interstices offering well-marked lateral spaces; the border of the mantle regularly scaly. _C. marginatus._ The marginated Chiton. Pl. 1, fig. 2. Valves carinated and projecting over each other; finely shagreened, with a dusky reddish-brown margin. _C. fascicularis._ The fasciculated Chiton. Pl. 1, fig. 3. Valves more narrow, imbricated, without distinct spaces; lateral parts of the skin naked or hairy, but always provided with silky or hairy tufts disposed in pairs between the junctions of the valve. _C. spinosus._ The spiny Chiton. Pl. 1, fig. 4. Shell beset with long, thin, curved, tubular, hairy, blackish spines. 4. Patella. The Limpet or Dishlike Shell. Forty-five species. The Patella of Linnæus was divided by Lamarck into several distinct genera, and now comprehends only such shells as are of a conical form, with an imperforated summit. The anterior is that part to which the summit inclines, and is always more narrow than the posterior part. It derives its name from its resemblance to a little dish; the colour and structure are various; some are smooth, others granulated, and many are covered with elevated tuberculated ribs. The exterior is sometimes of a pale fawn colour, and the interior of a bright pink; some have a silvery hue, but the more general colour is bluish white or light brown.