SELACHOIDEI. 325 



ISToTiDANUS. — Dentition unequal in the jaws : in the upper 

 jaw one or two pairs of awl-shaped teeth, the following six being 

 broader, and provided with several cusps, one of which is much 

 the strongest. Lower jaw with six large comb-like teeth on each 

 side, beside the smaller posterior teeth. Spiracles small, on the 

 side of the neck. No pit at the root of the caudal fin. Gill- 

 openings wide, six in number in Hexanchus, seven in Heptan- 

 cltus. 



Four species are known, distributed over nearly all the 

 tropical and sub-tropical seas ; tliey attain to a length of 

 about fifteen feet. Fossil teeth belonging to this type have 

 been found in Jurassic and later formations {Notidanus and 

 Aellopos). 



Fifth Family — Scylliid^. 



Tivo dorsal fins, without spine : the fi,rst above or behind 

 the ventrals ; anal fin present. No nictitating membrane. 

 Sp)iraele always distinct. Mouth inferior. Teeth smcdl, several 

 series generally being iii function. 



ScYLLiUM. — The origin of the anal fin is always in advance 

 of that of the second dorsal. Nasal cavity separate from the 

 mouth. Teeth small, with a middle longer cusp, and generally 

 one or two small lateral cusps arranged in numerous series. Eggs 

 similar to those of the Rays (Fig. 79, p. 167). 



The fishes of this genus are of small size, and commonly 

 called " Dog-fishes." They are coast fishes, living on the 

 bottom, and feeding on Crustaceans, dead fishes, etc. None 

 of the eight species known have a very wide distribution, but 

 where they occur they are generally sufficiently abundant to 

 prove troublesome to fishermen. They inhabit most parts of 

 the temperate and tropical seas. On the British coasts two 

 species are found, the " Larger" and " Lesser spotted Dog-fish," 

 Scylliuni canieula and ScylUtim catulus, wdiich are said to be 

 more plentiful among the Orkney Islands than elsewhere. 

 They are scarcely ever brought to market ; but the fishermen 

 of some localities do not disdain to eat them. Their flesh is 



