xvii ELASMOBRANCHII SELACHII 451 



without spines. Tail with a prominent lateral keel on each 

 side. Nictitating membranes absent.- Spiracles minute or 

 wanting. Branchial clefts very wide. No oro- nasal grooves. 

 Vertebrae asterospondylic. "When fully developed the teeth are 

 solid. 



In the genus Lamna, which includes the Porbeagle Sharks, 

 the teeth are large, each consisting of a long narrow central cusp, 

 usually with smaller cusps at the base. The common Porbeagle 

 (L. cornulica}, a fierce pelagic Shark, which may reach a length of 

 10 feet, frequents the North Atlantic and the North Pacific 

 (Fig. 257). It has often been captured off the coasts of 

 Great Britain and Ireland in Mackerel or Salmon nets, or 

 by lines laid for food Fishes. An allied genus, Isurus, is 



FIG. 257. The Common Porbeagle (Lamna cornubica). (From Parker and Haswell, 

 after Bashford Dean.) 



represented by species on the Atlantic coast of North America, 

 in the Mediterranean and the neighbouring parts of the 

 Atlantic, and also in Asiatic seas. Carcharodon rondeletii * is a 

 pelagic Shark with large, triangular, finely-serrated teeth, without 

 basal cusps, and is found in all tropical and subtropical seas from 

 the Mediterranean to Australia and New Zealand. It is one 

 of the largest and most formidable of Sharks, and it is said to 

 grow to a length of 40 feet. Nothing is known of its breeding 

 habits. Odontaspis, which has minute pore-like spiracles, but 

 no lateral caudal keels, is a Shark of moderate size, chiefly 

 inhabiting the Atlantic, but found also in the Mediterranean 

 and the Southern Pacific. Its teeth are long and awl-like, with 

 small basal cusps. 



The Thresher or Fox Shark (Alopecias vulpes) is remarkable 

 for the extraordinary length of the upper lobe of the caudal fin, 

 1 T. J. Parker, P.Z.S. 1887, p. 27. 



