Fig. 317.—Teeth of Cestraciont Sharks. (After Woodward.) d, Synechodus dubrisianus Mackie; e, Heterodontus canaliculatus Egerton; f, Hybodus striatulus Agassiz. (After Woodward.)
Fig. 318.—Egg of Port Jackson Shark, Heterodontus philippi (Lacépède). (After Parker & Haswell.)
Family Heterodontidæ.—Among recent species this group contains only the family of Heterodontidæ, the bullhead sharks, or Port Jackson sharks. In this family the head is high, with usually projecting eyebrows, the lateral teeth are pad-like, ridged or rounded, arranged in many rows, different from the pointed anterior teeth, the fins are large, the coloration is strongly marked, and the large egg-cases are spirally twisted. All have five gill-openings. The living species of Heterodontidæ are found only in the Pacific, the Port Jackson shark of Australia, Heterodontus philippi, being longest known. Other species are Heterodontus francisci, common in California, Heterodontus japonicus, in Japan, and Heterodontus zebra, in China. These small and harmless sharks at once attract attention by their peculiar forms. In the American species the jaws are less contracted than in the Asiatic species, called Heterodontus. For this reason Dr. Gill has separated the former under the name of Gyropleurodus. The differences are, however, of slight value. The genus Heterodontus first appears in the Jurassic, where a number of species are known, one of the earliest being Heterodontus falcifer.
Three families of Cestraciontes are recognized by Hay. The most primitive of these is the group of Orodontidæ. Orodus, from the Lower Carboniferous, has the teeth with a central crown, its surface wrinkled. Of the Heterodontidæ, Hybodus, of the Carboniferous and Triassic, is one of the earliest and largest genera, characterized by elongate teeth of many cusps, different in different parts of the jaw, somewhat as in the Hexanchidæ, the median points being, however, always longest. The dorsal fins are provided with long spines serrated behind. The vertebræ with persistent notochord show qualities intermediate between those of Hexanchidæ and Heterodontidæ, and the same relation is shown by the teeth. In this genus two large hooked half-barbed dermal spines occur behind each orbit.
Fig. 319.—Tooth of Hybodus delabechei Charlesworth. (After Woodward.)
Fig. 320.—Fin-spine of Hybodus basanus Egerton. Cretaceous. Family Heterodontidæ. (After Nicholson.)
