448 FISHES CHAP. 



its lateral sensory canals in a series of incomplete calcified rings, 

 as in the Holocephali. 



Fam. 8. Carchariidae. Sharks with two dorsal fins, 

 the first in front of the pelvic fins and the second opposite the 

 anal fin, both devoid of spines. Tail without lateral keels. 

 Preoral rostrum elongated. Mouth crescentic. Eyes with 

 nictitating membranes. Spiracles small or absent. Vertebrae 

 asterospoiidylic. Teeth usually consisting of a single triangular 

 cusp, with smooth, trenchant, or serrated margins, rarely with 

 basal cusps ; generally with an axial cavity when fully developed. 

 Viviparous. The family comprises about twenty genera, and 

 approximately sixty species ; found in all seas, often in mid-ocean. 

 Amongst the more important genera may be mentioned Carcharias 

 (Carcharliinus}, Galeocerdo, Triakis, Thalassorhinus, Galeus, Mus- 

 telus and Scylliogaleus. 



FIG. 255. The Blue Shark (Carcharias glaucus}. (From Miiller and Henle.) 



Species of Carcharias are found in nearly all tropical and sub- 

 tropical seas. The genus is a somewhat comprehensive one, and 

 groups of its species have been distinguished as sub-genera under 

 the names of Prionodon, Hypoprion, Scoliodon, Aprionodon, 1 etc. 

 One of the most widely distributed of the thirty to forty species is 

 the Blue Shark, C. (Prionodon} glaucus (Fig. 255), of the Atlantic 

 and Pacific Oceans, which may grow to a length of 25 feet, although 

 the young forms not infrequently captured in British waters do 

 not exceed 6 to 8 feet. It is a slender, swift, pelagic Shark, of a 

 slaty-blue colour above, and white underneath, and a voracious 

 hunter of other Fishes. C. nicaraguensis, a Shark about 7 feet 

 long, is confined to Lake Nicaragua and its outlet the Rio San 

 Juan, and is one of the very rare strictly freshwater Sharks. 

 Galeocerdo is a large Shark found in temperate and tropical 

 waters, but one species, G. arcticus, is confined to Arctic seas. 

 The variegated G. tigrinus, or West Indian. Tiger-Shark, is said 

 1 Miiller and Henle, op. cit. 



