xii PHYLUM CHORDATA 389 



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 neigh- 

 bourhood, these Torpedo Rays are able to ward off the 

 attacks of enemies and to kill or paralyse their prey. 



Teeth are developed in all on the upper jaw or palato-quad- 

 rate and on the lower jaw or Meckel's cartilage. They are 



fit. orb 



FIG. 218. Lateral view of the skull of Heptanchus. nick. Meckel's cartilage ; 

 pal. gu. palato-quadrate ; pt. orb. post-orbital process of the cranium, with which 

 the palato-quadrate articulates. (After Gegenbaur.) 



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, or 

 triangular with serrated edges, or made up of several sharp 

 cusps ; in the Rays, however, the teeth are more or less 

 obtuse, sometimes, as in the Eagle Rays, forming a 



