392 MANUAL OF ZOOLOGY sect. 



connected with the skull through the intermediation of a 

 hyomandibular cartilage, or proximal element of the hyoid 

 arch ; in the great majority this is the sole articulation of 

 the jaws with the skull posteriorly, and the skull is on that 

 account said to be hyostylic ; but in Hexanchus and Hep- 

 tanchus (Fig. 234) the upper jaw has a direct articulation 

 with the skull behind the orbit, and the arrangement is 

 termed amphistylic. There are always five pairs of bran- 

 chial arches, except in Hexanchus and Chlamydoselachus, 

 which have six, and Heptanchus, in which there are seven. 



The basal cartilages of the pectoral fin are typically three, 

 as in the dogfish, but there are sometimes four, and the 

 number may be reduced to two. There are usually two 

 such cartilages in the pelvic fin, and one alone may be 

 present. 



Electric organs — organs in which electricity is formed 

 and stored up, to be discharged at the will of the fish — 

 occur in several Elasmobranchs. They are best developed 

 in the electric rays {Torpedo and Hypnos) in which they 

 form a pair of large masses running through the entire thick- 

 ness of the body between the head and the margin of the 

 pectoral fin. By means of the electric shocks which they 

 are able to administer at will to animals in their immediate 

 neighbourhood, these torpedo rays are able to ward off the 

 attacks of enemies and to kill or paralyse their prey. 1 



Teeth are developed in all on the palato-quadrate or 

 upper jaw and Meckel's cartilage or the lower jaw. They 

 are arranged in several parallel rows, and are developed 

 from a groove at the back of the jaw, successive rows 

 coming to the front, and, as they become worn out, falling 

 off and becoming replaced by others. In the sharks the 

 teeth are usually large and may be long, narrow, and pointed, 



1 Torpedo occidentalis occurs on the southern coast of New England. 



