446 FISHES CHAP. 



Probably some ichthyodorulites described under various generic 

 names belong to this family. 



Fam. 5. Psammodontidae. --Teeth large, flat or slightly 

 arched, oblong or quadrate, and arranged in one, two, or more 

 longitudinal rows. Only the teeth are known, and from differ- 

 ences in their shape, size, and surface markings, the genera 

 Psammodus, Arclieobatis, and Copodus have been recognised. 

 The family is confined to the Lower Carboniferoiis of Great 

 Britain and Ireland, Russia, Belgium, and North America. 



Fam. 6. Petalodontidae. -Teeth transversely elongated, with 

 a blunt or a sharply-ridged crown, separated from a single or 

 multiple root by a constricted neck, and disposed in transverse 

 and longitudinal pavement-like rows ; exoskejeton of smooth, oval, 

 rounded or quadrate shagreen denticles. Only the teeth, and in 

 some genera the dermal denticles, are known, except in Janessa, 

 which has a Ray-shaped body, with large pectoral fins prolonged 

 towards the head. The family is mainly confined to the Car- 

 boniferous formations of Great Britain, Europe, and North 

 America. Petalodus, Janessa (also represented in the Permian), 

 Glossodus, Polyrhizodus, and Callopristodus are characteristic 

 genera. 



Fam. 7. Scylliidae (Dog -Fishes). Dorsal fins two in 

 number, small, and without spines, the first above or behind 

 the pelvic fins, the second usually behind the anal. Tail 

 not bent upwards or but slightly so, without lateral keels. 

 Spiracles present. Nictitating membranes absent. Vertebrae 

 asterospondylic. Teeth small, each with a median cusp, and 

 one to four small cusps on each side. Oviparous. Egg -cases 

 (Fig. 246) large, quadrate, with long twining tendrils at the 

 angles for attachment. 



The genus Scyllium, includes the true Dog-Fishes (Fig. 254). 

 The species are coast Fishes of small or moderate size, and are 

 widely distributed in temperate and tropical seas, at depths not 

 as a rule exceeding 400 fathoms. Two species, S. canicula and 

 S. catulus, are common on the British coasts, living near the 

 bottom and feeding on Crustaceans and Molluscs. An allied 

 form, Pristiurus, is also common in European and British waters. 

 Ckiloscyllium is a widely-distributed, genus ranging from the 

 Cape of Good Hope through the Indian Ocean to the coasts 

 of Australia, China, and Japan. Stegostoma tigrinum of the 



