CHITON. A group of mollusks, the mail shells. constituting an order (Plaeophora) in the class Amphineura. The shell is composed of eight narrow, transverse, calcareous pieces, overlapping each other in a row along the back, and strongly attached to the mantle. which is remarkably fleshy and fibrous. Chitons have the power of rolling themselves up into a ball like the armadillo (q.v.). thus exposing nothing but the hard shell. The organ of locomotion is an oval foot, extending the whole length of the animal, by means of which ehitons cling to rocks so tenaciously that the heaviest surf does not disturb them. :More than 200 species arc known; they dwell in all climates, but are most abun dant in the warmer seas. They occur at all depths, but prefer the rocks along the shore, where they sometimes are found in surprising numbers. All North Atlantic species are small, seldom an inch in length, but some of those found in the tropics, and on the coast of California, may be 8 or 10 inches long. Most of the chitons are gray or brown in color. hut some species are very handsomely marked with red, orange, yel low, green, and other colors. The upper surface may be comparatively rough, with and spines. In most eases it is nearly covered by
the shell plates. but in one or two genera the plates arc very narrow, and cover only the mid dorsal part of the animal, while in still others the plates are completely concealed within the thickened integument. "West Indian negroes" are said to "eat the large chitons, which are abundant on their rocky coasts, cutting off and swallowing raw the fleshy foot, which they call beef." Fossil, Points. The modern chitons are the survivors of a long line of ancestors that made, their first appearance in the Ordovician or Lower Silurian rocks, attained some prominence during Carboniferous time, and, with a slight decline during the Tertiary period, have continued with only slight changes of scarcely more than ge neric rank down to the present era. The group thus affords an excellent example of the per sistence of a generalized primitive type. The fossil genera are classified according to the form of the articulating facets of the valves and the proportions of the 'shell and its segments. See also PLACOPTIORA.