86 PORT JACKSON SHARKS 
pavement teeth that have been preserved. Their bygone 
réle 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). 
