xvn 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. 1 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 



FIG. 262. The Saw- Fish (1'ristis antiquorum). (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, 1 

 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. Bouleiiger for these observations. 



