The oolitic genus Spathobatis is scarcely distinct from Rhinobatus; and another fossil from Mount Lebanon has been actually referred to this latter genus. Trigorhina from Monte Postale must be placed here.
Third Family—Torpedinidæ.
The trunk is a broad, smooth disk. Tail with a longitudinal fold on each side; a rayed dorsal fin is generally, and a caudal always, present. Anterior nasal valves confluent into a quadrangular lobe. An electric organ composed of vertical hexagonal prisms between the pectoral fins and the head.
“Electric Rays.” The electric organs with which these fishes are armed are large, flat, uniform bodies, lying one on each side of the head, bounded behind by the scapular arch, and laterally by the anterior crescentic tips of the pectoral fins. They consist of an assemblage of vertical hexagonal prisms, whose ends are in contact with the integuments above and below; and each prism is subdivided by delicate transverse septa, forming cells, filled with a clear, trembling, jelly-like fluid, and lined within by an epithelium of nucleated corpuscles. Between this epithelium and the transverse septa and walls of the prism there is a layer of tissue on which the terminations of the nerves and vessels ramify. Hunter counted 470 prisms in each battery of Torpedo marmorata, and demonstrated the enormous supply of nervous matter which they receive. Each organ receives one branch of the Trigeminal nerve and four branches of the Vagus, the former, and the three anterior branches of the latter, being each as thick as the spinal chord (electric lobes). The fish gives the electric shock voluntarily, when it is excited to do so in self-defence or intends to stun or to kill its prey; but to receive the shock the object must complete the galvanic circuit by communicating with the fish at two distinct points, either directly or through the medium of some conducting body. If an insulated frog’s leg touches the fish by the end of the nerve only, no muscular contractions ensue on the discharge of the battery, but a second point of contact immediately produces them. It is said that a painful sensation may be produced by a discharge conveyed through the medium of a stream of water. The electric currents created in these fishes exercise all the other known powers of electricity: they render the needle magnetic, decompose chemical compounds, and emit the spark. The dorsal surface of the electric organ is positive, the ventral surface negative.
[The literature on the electric organ of Torpedo is very extensive. Here may be mentioned Lorenzini, “Osservazioni intorno alle Torpedini,” (1678); Walsh, “On the Electric Property of the Torpedo,” in Philos. Trans., 1773; Hunter, “Anatomical Observations on the Torpedo,” ibid.; Davy, “Observations on the Torpedo,” in Philos. Trans., 1834; Matteucci and Savi, “Traité des Phénomènes Electro-Physiologiques,” 1844.]
Of the genus Torpedo six species are known, distributed over the Atlantic and Indian Oceans; three of them are rather common in the Mediterranean, and one (T. hebetans) reaches the south coast of England. They attain to a width of from two to three feet, and specimens of that size are able to disable by a single discharge a full-grown man, and, therefore, may prove dangerous to bathing persons. Other genera, differing from Torpedo in the position and structure of some of the fins, are found in other tropical and sub-tropical seas, viz. Narcine, Hypnos, Discopyge (Peru), Astrape, and Temera. All, like electric fishes generally, have a naked body.
A large fish, of the general appearance of a Torpedo, has been found at Monte Bolca; and Cyclobatis, from the upper cretaceous limestone of Lebanon, is probably another extinct representative of this family.
Fourth Family—Rajidæ.
Disk broad, rhombic, generally with asperities or spines; tail with a longitudinal fold on each side. The pectoral fins extend to the snout. No electric organ; no serrated caudal spine.
Raja.—Two dorsal fins on the tail, without spine; tail with a rudimentary caudal fin, or without caudal. Each ventral fin divided into two by a deep notch. Teeth small, obtuse, or pointed. Pectoral fins not extending forwards to the extremity of the snout. Nasal valves separated in the middle, where they are without a free margin (see Fig. [1], p. 34).