=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.