Narcobatidæ, or Torpedoes.—The torpedoes, or electric rays (Narcobatidæ), are characterized by the soft, perfectly smooth skin, by the stout tail with rayed fins, and by the ovoviviparous habit, the eggs being hatched internally. In all the species is developed an elaborate electric organ, muscular in its origin and composed of many hexagonal cells, each filled with soft fluid. These cells are arranged under the skin about the back of the head and at the base of the pectoral fin, and are capable of benumbing an enemy by means of a severe electric shock. The exercise of this power soon exhausts the animal, and a certain amount of rest is essential to recovery.
The torpedoes, also known as crampfishes or numbfishes, are peculiarly soft to the touch and rather limp, the substance consisting largely of watery or fatty tissues. They are found in all warm seas. They are not often abundant, and as food they have not much value.
Perhaps the largest species is Tetronarce occidentalis, the crampfish of our Atlantic coast, black in color, and said sometimes to weigh 200 pounds. In California Tetronarce californica reaches a length of three feet and is very rarely taken, in warm sandy bays. Tetronarce nobiliana in Europe is much like these two American species. In the European species, Narcobatus torpedo, the spiracles are fringed and the animal is of smaller size. To Narcine belong the smaller numbfish, or "entemedor," of tropical America. These have the spiracles close behind the eyes, not at a distance as in Narcobatus and Tetronarce. Narcine brasiliensis is found throughout the West Indies, and Narcine entemedor in the Gulf of California. Astrape, a genus with but one dorsal fin, is common in southern Japan. Fossil Narcobatus and Astrape occur in the Eocene, one specimen of the former nearly five feet long. Vertebræ of Astrape occur in Prussia in the amber-beds.
Fig. 346.—Teeth of Janassa linguæformis Atthey. Carboniferous. Family Petalodontidæ. (After Nicholson.)
Petalodontidæ.—Near the Squatinidæ, between the sharks and the rays, Woodward places the large extinct family of Petalodontidæ, with coarsely paved teeth each of which is elongate with a central ridge and one or more strong roots at base. The best-known genera are Janassa and Petalodus, widely distributed in Carboniferous time. Janassa is a broad flat shark, or, perhaps, a skate, covered with smooth shagreen. The large pectoral fins are grown to the head; the rather large ventral fins are separated from them. The tail is small, and the fins, as in the rays, are without spines. The teeth bear some resemblance to those of Myliobatis. Janassa is found in the coal-measures of Europe and America, and other genera extend upward from the Subcarboniferous limestones, disappearing near the end of Carboniferous time. Petalodus is equally common, but known only from the teeth. Other widely distributed genera are Ctenoptychius and Polyrhizodus.
Fig. 347.—Polyrhizodus radicans Agassiz. Family Petalodontidæ. Carboniferous of Ireland. (After McCoy.)
These forms may be intermediate between the skates and the sting-rays. In dentition they resemble most the latter.
Similar to these is the extinct family of Pristodontidæ with one large tooth in each jaw, the one hollowed out to meet the other. It is supposed that but two teeth existed in life, but that is not certain. Nothing is known of the rest of the body in Pristodus, the only genus of the group.