No nictitating membrane. Two dorsal fins, the first opposite to the space between pectoral and ventral fins; anal fin present. Nasal and buccal cavities confluent. Teeth obtuse, several series being in function.
Fig. 119.—Jaws of Port Jackson Shark, Cestracion philippi.
Fig. 120.—Upper jaw of the same, half natural size.
This family is one of particular interest, because representatives of it occur in numerous modifications in primary and secondary strata. Their dentition is uniformly adapted for the prehension and mastication of crustaceous and hard-shelled animals. The fossil forms far exceeded in size the species of the only surviving genus; they make, their appearance with Ctenoptychius in the Devonian; this is succeeded in the coal-measures by Psammodus, Chomatodus, Petrodus, Cochliodus, Polyrhizodus, etc.; in the Trias and Chalk by Strophodus, Acrodus, Thectodus, and Ptychodus. Of the 25 genera known, 22 have lived in the periods preceding the Oolitic.
Cestracion (Heterodontus).—Each dorsal fin armed with a spine in front; the second in advance of the anal. Mouth rather narrow. Spiracles small, below the posterior part of the eye. Gill-openings rather narrow. Dentition similar in both jaws, viz. small obtuse teeth in front, which in young individuals are pointed and provided with from three to five cusps. The lateral teeth are large, padlike, twice as broad as long, arranged in oblique series, one series being formed by much larger teeth than those in the other series.
Fig. 121.—Cochliodus contortus.
