526 



The True Sharks 



developed, with the three basal cartilages; there are five gill- 

 openings, and the tail is heterocercal. 



FIG. 315. Bullhead-shark, Heterodontus francisci (Girard). San Pedro, Cal. 



Suborder Cestraciontes. The most ancient types may be set 

 off as a distinct suborder under the name of Cestraciontes or 

 Prosarthri. 



FIG. 316. Lower jaw of Heterodontus philippi. From Australia. Family Hetero- 



dontidcB. (After Zittel.) 



These forms find their nearest allies in the Notidani, which 

 they resemble to some extent in dentition and in having the 

 palato-quadrate articulated to the skull although fastened 



