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 diffeér- 
ences in their shape, size, and surface markings, the genera 
Psammodus, Archeobatis, and Copodus have been recognised. 
The family is confined to the Lower Carboniferous 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 ; exoskeleton 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 Seylliwm 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 Molluses. An allied 
form, Pristivrus, is also common in European and British waters. 
Chiloscyllium 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 
