Three species are known from the Australian and Japanese coasts. They are evidently ground-sharks, which lie concealed on the bottom watching for their prey. In accordance with this habit their colour closely assimilates that of a rock or stone covered with short vegetable and coralline growth--a resemblance increased by the frond-like tentacles on the side of the head. This peculiarity of the integuments, which is developed in a yet higher degree in Pediculati and Lophobranchs, is not met with in any other Selachian. These Sharks grow to a length of 10 feet. SIXTH FAMILY--HYBODONTIDÆ. _Two dorsal fins, each with a serrated spine. Teeth rounded, longitudinally striated, with one larger, and from two to four smaller lateral cusps. Skin covered with shagreen._ Extinct. From carboniferous, liassic, and triassic formations. Several genera have been distinguished; and if _Cladodus_ belongs to this family, it would have been represented even in the Devonian. [Illustration: Fig. 118.--Spine of Hybodus subcarinatus.] SEVENTH FAMILY--CESTRACIONTIDÆ. _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._ [Illustration: Fig. 119.--Jaws of Port Jackson Shark, Cestracion philippi.] [Illustration: 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_, _Coch__liodus_, _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. [Illustration: Fig. 121.--Cochliodus contortus.] [Illustration: Fig. 122.--Cestracion galeatus, Australia.] Four species are known from Japan, Amboyna, Australia, the Galapagoes Islands, and California; none exceed a length of 5 feet. The egg has been figured on p. 168 (Fig. 80).