36 PORT JACKSON SHARKS 



pavement teeth that have been preserved. Their bygone 

 role was certainly a long and important one. In some 

 of their forms they could have differed but little from 

 their single survivor, the Port Jackson shark, Cestracion 

 (Heterodontus) (Fig. 91, A, B, C). In others, the denti- 

 tion and dermal defences suggest a wide range in evo- 

 lution. Their general character appears to have been 

 primitive, but in structural details they were certainly 

 specialized ; thus their dentition had become adapted to a 

 shellfish diet, and they had evolved defensive spines at 

 the fin margins, sometimes even at the sides of the head. 

 In some cases the teeth remain as primitive shagreen 

 cusps on the rim of the mouth, but become heavy and 

 blunted behind ; in other forms the fusion of tooth clus- 

 ters may present the widest range in their adaptations for 

 crushing ; and the curves and twistings of the tritoral sur- 

 faces may have resulted in the most specialized forms of 

 dentition (e.g. Janassas, Petalodonts, Cochliodonts, and Psam- 

 modonts of the Coal Measures) which are known to occur 

 not merely in sharks but among all vertebrates. Equally 

 interesting may prove the evolutional details of other 

 cestraciont structures when they come to be known. 

 Ray-like proportions may well have been evolved even 

 among the earliest Palaeozoic forms. 



The surviving member of this group, Cestracion, sug- 

 gests in itself the adaptations of a bottom-living form in 

 its greatly enlarged pectorals. Its genus, however, has 

 not been traced earlier than the Mesozoic, although its 

 comparatively generalized dentition (Fig. 27) suggests a 

 far more remote descent. 



It is of interest to note that Cladoselache approaches in 

 its dentition the characters of the primitive Cestracionts 

 (e.g. Synechodus). 



