The True Sharks 187 
developed, with the three basal cartilages; there are five gill- 
openings, and the tail is heterocercal. 
Fig. 129.—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 
Prosarthrt. 
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Fic. 130.—Lower jaw of Heterodontus philippi. From Australia. Family Hetero- 
dontide. (After Zittel.) 
These forms find their nearest allies in the Notidant, which 
they resemble to some extent in dentition and in having the 
palato-quadrate articulated to the skull although fastened 
