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 Zamna, 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. cornubica), 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, Jswrus, 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, 
Dee bar kerr. as loots) Dalal. 
