SHARKS AND RA YS. 



521 



Fig. 39.— Thoenback Skate. 



Family 



Torpedinidm. 



of the electric 



the typical genus belongs the common thornback skate of the British seas, 

 in which the two sexes are distin- 

 guished by the structure of the teeth, 

 those of the female forming a com- 

 paratively smooth pavement, whereas 

 those of the male — although still 

 arranged in the same manner — are 

 cusped. In other species of the 

 same genus sexual differences dis- 

 play themselves in other parts of 

 the body. Skates of this genus 

 are more abundant in northern than 

 in tropical seas, and some of them 

 range considerably farther north than 

 any other members of the group. 

 The flesh of all is largely used as 

 food. 



The presence of an electric organ, 

 composed of vertical prisms, between the head and the pectoral fins, serves 

 to differentiate the numerous kinds of electric rays from the 

 other representatives of the group. The body of these fishes 

 forms a broad, smooth disc, covered with a naked skin ; the 

 tail has a longitudinal fold on each side and a caudal fin, and 

 there is generally a rayed dorsal fin. The vertical prisms 

 organ are divided by a number of horizontal partitions into separate cells, 

 each filled with a translucent substance somewhat resembling jelly in ap- 

 pearance. The currents produced by this singular structure are precisely 

 similar to those from a galvanic battery ; and in fishes of moderate dimensions 

 are sufiiciently powerful to disable even human beings. The typical genus 

 Tm-pedo includes half a dozen species from the Indian and Atlantic Oceans, 

 some of which are also abundant in the Mediterranean, while one occasionally 

 wanders to the southern shores of Britain. From the warmer seas there are 

 also several other genera belonging to this family, such as Astrape, Biscopyge, 

 Hypius, Narcine, and Temera, severally distinguished by the conformation 

 and position of the fins. 



The largest and most hideous of all the skates are the so-called eagle-rays, 

 several of which are popularly known as devil-fish. In the five genera in- 

 cluded in this family the pectoral fins obtain an enormous 

 development, in consequence of which the disc becomes Family 



greatly widened. Along the sides of the head the pectoral fins MyliobatidcE. 

 are, however, interrupted ; their anterior extremities form- 

 ing small finlets on tlie muzzle, known as cephalic fins. In some of the 

 genera these cephalic fins are modified to form a pair of horn-like structures 

 projecting forwards on each side of the muzzle, the function of which is to 

 assist in capturing and conveying to tlie mouth the prey. In all, the tail 

 forms a long, tapering, whip-like organ. When the mouth is armed with 

 teeth, these take the form of a pavement, which in tlie typical genus Myho- 

 batis is perfectly smooth, and composed of a number of oblong and hexagonal 

 plates, fitted closely together at their edges, and with their grinding surface 

 like polished ivory. Several widely-spread species are included in the 

 typical genus, two of which are visitors to the British coasts. Most of them 

 have a barbed spine to the tail, and with this, when caught, they lash out 



