XVII ELASMOBRANCHII—-BATOIDEI 459 
Selachii. In others, however, the hinder gill-clefts tend to 
extend backwards above the base of the pectoral fins, while in 
some the clefts assume a more ventral position, and extend 
beneath the pectoral fin; hence, even within the limits of the 
Selachii the position of the gill-clefts varies to the extent that 
these structures may be lateral, or they may tend to become 
either dorsal or ventral.’ On the score of convenience the 
customary usage is adopted here. ; 
Fam. 1. Pristidae (True Saw-Fishes).—Although somewhat 
depressed, the body is still elongate and Shark-like, with a well- 
developed tail terminating in a heterocercal caudal fin. Dorsal 
fins large, the first opposite the pelvic fins. Head and skull pro- 
longed into a long flattened rostrum, the lateral margins of which 
are armed with a series of strong tooth-like denticles, firmly im- 
planted in sockets in the calcified rostral cartilage. No rostral 
Fic. 262.—The Saw-Fish (Pristis antiquorwm). (From Cuvier.) 
tentacles. Teeth in the jaws minute and obtuse. One genus 
and about four or five species are known, all inhabitants of 
tropical and subtropical seas. 
Some of the true Saw-Fishes attain a considerable size, 10 
to 20 feet or even longer, and “saws” 6 feet long and a foot 
in. width across the base are not uncommon. By means of 
powerful lateral strokes of its saw the Fish is capable of 
lacerating the bodies of other animals and tearing off pieces 
of flesh, which it then devours. Indian species are known to 
ascend rivers beyond tidal influence, and an American species, 
ranging northwards to the West Indies and the Gulf of Mexico, 
where it is abundant, enters the lower Mississippi. P. anti- 
quorum occurs in the Mediterranean and the Atlantic, but does 
not extend so far northward as the British coasts. 
The earliest known representative of the family is the 
1 I am indebted to Mr. Boulenger for these observations. 
