ELASMOBRANCHII. 423 



tures. The flattened bodies of the Rays, however, must be 

 carefully distinguished from those of the Flat-fishes (Pleuronec- 

 tida). In the former the 

 flat surfaces of the body are 

 truly the dorsal and ventral 

 surfaces. In the latter, as 

 before remarked, the body 

 is flattened, not from above 

 downwards, but from side to 

 side, and the head is so 

 twisted that both eyes are 

 brought to one side of the 

 body. The tail in the Rays 

 is long and slender, usually 

 armed with spines, and gene- 

 rally with two or three fins 

 (the homologues of the dor- 

 sal fins). The mouth is paved 

 with flat teeth of a more or 

 less rhomboidal shape. 



The typical members of the 

 Batides are the Skates and Fig . l8l ._ Batides . Raia ma r g inata, one of 



RayS, Of which the Common the Skates. Reduced one-sixth. (After 



Thornback (Raia clavatd) 



may be taken as a familiar example. More remarkable than the 

 common Rays is the Electric Ray or Torpedo, which has the 

 power of discharging electrical shocks, if irritated. The identity 

 of the force produced in this way with the electricity of the 

 machine has been demonstrated by many careful experiments. 

 The Torpedo owes its remarkable powers to two special organs 

 the "electrical organs," which consist of two masses placed on 

 each side of the head, and consisting each of numerous vertical 

 gelatinous columns, separated by membranous septa, and richly 

 furnished with nerves from the eighth pair ; the whole arrange- 

 ment presenting a singular resemblance to the cells of a voltaic 

 battery. There is no doubt, however, but that the force which 

 is expended in the production of the electricity is only nerve- 

 force. For every equivalent of electricity which is generated, 

 the fish loses an equivalent of nervous energy ; and for this 

 reason the production of the electric force is strictly limited 

 by the amount of nerve-force possessed by the animal. 



Other well-known members of the family are the Sting-rays 

 (Trygon), the Eagle-rays (Myliobatis], the Horned Rays (Cepha- 

 lopterd), and the Beaked Rays (Rhinobatis}. 



In the Saw-fish (Pristis antiquorwri) the body has not the 



