PORT JACKSON SHARK. 



29 



existed in Carboniferous times and were more numerous in 

 Palaeozoic and Mesozoic periods than at the present day. 



In Cestracion the mouth is rather narrow and nearly terminal. 

 The spiracle is small and situated below the posterior part of the 

 eye; the gill openings are rather small. The dorsal fin-spines 

 are smooth and with no posterior serrations such as occur in 

 Hybodus (see spine, 21, and fig. 15). The jaws are suspended by 

 the hyomandibular cartilage, but the upper jaw 

 also enters into extensive articulation with the 

 ethmoid region of the cranium (see skull of 

 Cestracion galeatus, 17; the more usual type 

 of hyostylic skull found in Sharks is illustrated 

 by a skull of Scyllium, 18). The vertebral 

 centra of Cestracion are asterospondylic (see 

 20), but the radiating arrangement of the 

 secondary lamina? of calcareous matter does 

 not occur in the more ancient genera of the 

 family, e. g. Hybodus and Palceospinax. The 

 egg-shell of Cestracion has a curious spiral 

 flange projecting from its surface (see 6). 



The species of Cestracion occur in the seas 



of Australia, Japan, California, &c. The 



specimen 24 shown in Wall-case 1 is a small 



example of the Port Jackson Shark ; a larger 



specimen (four feet long) is shown in the 



Table-case 23, in the centre line of the 



Gallery ; a full-sized individual is about five 



feet long. For comparison with the teeth 



of Cestracion (19) are shown the teeth of the 



extinct Acrodus (22) and Asteracanthus (23). 



The Sharks of the family Cochliodontida? 



flourished in Carboniferous times, and their 



remains are practically confined to the rocks of 



that age. The dentition differs from that of 



the Cestraciontidse in one or more of the 



transverse series of teeth being fused into a continuous curved 



plate. Whereas in Cestracion the reserve members of the series 



of crushing teeth arise as separate teeth on the lingual or inner 



Fig. 15. — Dorsal fin- 

 spine of Hybodus. 

 (From Giinther, 

 "Study of Fishes.") 



Cochli 

 odus. 



