526 



The True Sharks 



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

 openings, and the tail is heterocercal. 



Fig. 315.— Bullhead-shark, Hcterodontii.^ 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 

 Prosartliri. 



Fig. 3)0. — Lower jaw of Hetfrodontus philippi. From Australia. Family Hetcro- 



dontida:. (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 



