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 
asterospondylic. 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 
(Carcharhinus), Galeocerdo, Triakis, Thalassorhinus, Galeus, Mus- 
telus and Scylliogaleus. 
Fia, 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,' ete. 
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. 
