244 THE PLAGIOSTOMIA. 



tooth having a single denticle at each side of its base. Eye moderate, pupil 

 erect. Spiracles small, a short distance behind the eye. Gill openings medium, 

 anterior farther apart, hindmost widest, in front of the pectoral. Pectorals short, 

 subtruncate, angles rounded. Dorsals and ventrals behind the mid total length; 

 ventrals the larger. Dorsals small, rounded; first dorsal inserted above the 

 middle of the bases of the ventrals; second little smaller than the first, similar 

 in shape, behind the ventrals, not reaching the caudal. Caudal about one 

 fourth of the total length, vertebral axis rising backward. Subcaudal fin rather 

 deep, without a distinct lobe, conca^-e on the hind margin, separated from the 

 terminal by a shallow notch,-— not always present; terminal pointed. Bucklers 

 scattered, irregular, radiate based, surmounted by a sharp spine. Said to reach 

 a length of nine feet or more. 



Back brown, tinted with purple or violet, with or without blotches of 

 darker; lower surfaces lighter to white. 



Tropical and temperate Atlantic and Pacific; Mediterranean; Australia. 



Pristiophoridae. 



This family contains small sharks remotely allied to the Squalidae and 

 distinguished from all others of the Antacea by the great depression of the head 

 and the very long blade-like snout, supported by the greatly produced rostral 

 and preorbital cartilages, at each edge of which there is a series of sharp teeth, 

 in some degree resembling the saw on the species of Pristis of the Platosomia. 



A barbel below each side of the lamina at some distance in front of the 

 nostrils. Nostrils inferior. Mouth below the head, behind the eyes; a rudi- 

 mentary labial fold at the angles on the lower jaws; teeth small, numerous, a 

 number of series in function at once. Eyes superior, elongate, with lateral 

 outlook; no nictitating membrane. Gill apertures lateral, in front of the 

 pectorals. Spiracles rather large, behind the eye. Dorsal fins well developed, 

 anterior above the body, posterior above the tail, not preceded by spines. No 

 anal fin. Tail with a dermal fold at the lower edge of each side. Subcaudal fin 

 reduced, separated from the tip by a notch; supracaudal fin broader backward. 



The genera included in the Pristiophoridae are so closely related that there 

 is little beside the number of gill openings to separate them. 



Gill openings five ...... Pristiophorus (page 245) 



Gill openings six Pliotrema (page 248) 



