PORT JACKSON SHARK. 



29 



existed in Carboniferous times and were more numerous in 

 Palaeozoic and Mesozoic periods than at the present day. 



In Cestraeion 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 

 Cestraeion galeatus, 17; the more usual type 

 of hyostylic skull found in Sharks is illustrated 

 by a skull of Scyttium, 18). The vertebral 

 centra of Cestraeion 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 Paleeospinax. The 

 egg-shell of Cestraeion has a curious spiral 

 flange projecting from its surface (see 6). 



The species of Cestraeion 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 Cestraeion (19) are shown the teeth of the 

 extinct Acrodus (22) and Aster acanthus (23). 

 The Sharks of the family Cochliodontidae 

 flourished in Carboniferous times, and their 

 remains are practically confined to the rocks of 

 that age. The dentition differs from that of 

 the Cestraciontidee in one or more of the 

 transverse series of teeth being fused into a continuous curved 

 plate. "Whereas in Cestraeion 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 Giintner, 

 "Study of Fishes.") 



Cochli- 

 odus.. 



