The sharks, skates, etc. (Elasmobranchii).—The sharks and skates are characterized by the possession of a skeleton composed of cartilage and not bone, as in the bony fishes; they have no operculum; their teeth are distinct, often large and highly specialized, and their eggs are few and very large. There are two principal groups among Elasmobranchii, viz., the sharks, which usually have an elongate body, and always have the gill-openings on the sides, and the rays or skates, which have a broad flattened body with the gill-openings always on the under side. All the members of both groups are marine. The sharks are active, fierce, usually large fishes, which live in the surface-waters of the ocean and make war on other marine animals, all of the species except half a dozen being fish-eaters. The shark's mouth is on the under side of the usually conical head, and the animal often turns over on its back in order to seize its prey. The largest American sharks, and the largest of all fishes, are the great basking-sharks (Cetorhinus), which reach a length of nearly forty feet. They get their name from their habit of gathering in numbers and floating motionless on the surface. They feed chiefly on fishes.

The hammer-headed sharks (Sphyrna) are odd sharks which have the head mallet or kidney shaped, twice as wide as long, the eyes being situated on the ends of the lateral expansions of the head. The man-eating or great white sharks (Carcharodon) are nearly as large as the basking-sharks, and are extremely voracious. They will follow ships for long distances for the refuse thrown overboard. They do not hesitate to attack man. Among the more familiar smaller sharks are the dog-fishes and sand-sharks of our Atlantic coast.

The rays and skates are also carnivorous, but are with few exceptions sluggish, lying at the bottom of shallow shore-waters. They feed on crabs, molluscs, and bottom-fishes. The small common skates, "tobacco-boxes" (Raja erinacea) (fig. [114]), about twenty inches long, and the larger "barn-door skates" (R. lævis), are numerous along the Atlantic coast from Virginia northward. Especially interesting members of this group, because of the peculiar character of the injuries produced by them, are the sting-rays and torpedoes or electric-rays. The sting-rays (Dasyatis) have spines near the base of the tail which cause very painful wounds. The torpedoes (Narcine) have two large electrical organs, one on each side of the body just behind the head, with which they can give a strong electric shock. "The discharge from a large individual is sufficient to temporarily disable a man, and were these animals at all numerous they would prove dangerous to bathers." Very different from the typical rays in external appearance are the saw-fishes (Pristis pectinatis) which belong to this group. The body is elongate and shark-like, and has a long saw-like snout. This saw, which in large individuals may reach a length of six feet and a breadth of twelve inches, makes its owner formidable among the small sardines and herring-like fishes on which it feeds. The saw-fishes live in tropical rivers, descending to the sea.

Fig. 114.—The common skate, Raja erinacea. (From Kingsley.)

The bony fishes (Teleostomi).—The bony or true fishes are distinguished from the lampreys and sharks and rays by having in general the skeleton bony, not cartilaginous, the skull provided with membrane bones, and the eggs small and many. In this group are included all the fishes of our fresh-water lakes, ponds, and streams as well as most of the marine forms. Fish life, being spent under water, is not familiar to most of us, and beginning students are rarely helped enough in getting acquainted with the different kinds and the interesting habits of fishes. But they offer a field of study which is really of unusual interest and profit. We can refer in the following paragraphs to but few of the numerous common and readily found kinds, and to these but briefly.

Closely related to the sunfish, studied as example of the bony fishes, are the various kinds of bass, as the "crappie" (Pomoxis annularis), the calico bass (P. separoides), the rock-bass (Ambloplites rupestris) and the large-mouthed and small-mouthed black bass (Micropterus salmoides and M. dolomieu respectively). All the members of this sunfish and bass family are carnivorous fishes especially characteristic of the Mississippi valley.

Another family of many species especially common in the clear, swift, and strong Eastern rivers is that of the darters and perches. The darters are little slender-bodied fishes which lie motionless on the bottom, moving like a flash when disturbed and slipping under stones out of sight of their enemies. Some are most brilliantly colored, surpassing in this respect all other fresh-water fishes.

Unlike the sunfishes and darters are the catfishes, composing a great family, the Siluridæ. The catfish (Ameiurus) gets its name from the long feelers about its mouth; from these feelers also come its other names of horned pout, or bull-head. It has no scales, but its spines are sharp and often barbed or jagged and capable of making a severe wound.