Most fishes have the body covered with scales, although many have the skin naked or covered with small scales so hidden in the skin as to be hardly visible. The scales are small horny or bony plates which fit into small pockets or folds of the skin, and are usually arranged shingle-fashion, overlapping each other. They are of various shapes, mostly classified as of three kinds, namely, squarish enamelled scales called ganoid, roundish smooth-edged called cycloid, and roundish tooth-edged called ctenoid. The skeleton of the fish is relatively complex. Its bones are comparatively soft, having little lime in them, indeed in many cases they are mere cartilage. The vertebral column is made of twenty-four vertebræ in the typical fishes, the number in the others being variously increased, or sometimes diminished. These vertebræ are of two classes, abdominal or body, and caudal or tail vertebræ. The former have a neural arch which encloses the spinal cord and from which projects a spine. Below, the processes spread apart, surrounding the kidneys and partly enclosing the air-bladder. To these processes ribs are loosely attached. The caudal vertebræ have no ribs and leave no room below for viscera. Their lower arch (hæmal), similar to the dorsal (neural) arch, surrounds a blood-vessel. The fins of a fish are composed of bony rods or rays joined by membrane. Some of these rays may be unbranched and unjointed, being then known as spines, and usually occupy the front part of the fin. Other rays are made up of little joints and are usually branched toward their tip. Such ones are called soft rays. Soft rays make up the greatest part of most fins. The vertical fins are on the middle line of the body. These are the dorsal above, anal below, and caudal forming the end of the tail. The paired pectoral and ventral fins are ranged one on each side corresponding to the arms and legs of higher animals. The pectoral fin or arm is fastened to a series of bones called the shoulder girdle. These bones do not correspond to those in the shoulder girdle of the higher animals, and the various parts in the two structures are differently named. The uppermost bone of the shoulder girdle is usually attached to the skull. To the lowermost is attached the rudimentary pelvis, which supports the hinder limb or ventral fin. Usually the pelvis is farther back and loose in the flesh, but sometimes it is placed far forward, being occasionally attached at the chin.