A Cestraphori. In this division there is a strong spine in front of each dorsal fin, and the back teeth are obtuse (fig. 270). The only living representatives of this group are the Port Jackson Shark (Cestracion Philippi), and some allied forms, characterised by their pavement of plate-like crushing teeth, adapted for comminuting small Molluscs and Crustaceans. They are exclusively inhabitants of the Pacific Ocean, and are remarkable for their close resemblance to a large group of extinct forms, of which the best known are the genera Hybodus and Acrodus from the Secondary rocks.

Fig. 270.   Upper jaw of Port Jackson Shark (Cestracion), showing the pavement of crushing teeth. One half the natural size. (After Owen.)

Fig. 270. - Upper jaw of Port Jackson Shark (Cestracion), showing the pavement of crushing teeth. One-half the natural size. (After Owen.)