220 USE OF TAIL OF THRESHER SHARK. 



passage, throngli wliicii the food— not without several 

 circumvohitions, and, in fact, by a long route — is con- 

 ducted to its exit. Here the shortness of the gut is 

 compensated by the obliquity of the perforation." This 

 structure can be seen in any of the common dog-fishes 

 or skates. 



In his Bridgwater Treatise my father applies this fact to the 

 elucidation of the intestinal structure of the monster Ichthyosauri 

 and Plesiosauri, which formerly existed in large numbers m the 

 British seas, and whose skeletons are so fiequently found fossil. 

 His argument was that, according to the structure of the teeth of 

 an animal, so would be the structure of the stomach ; and when 

 he found that the coprolites were marked externally with spiral 

 grooves, he concluded that the structure of the intestines of the 

 Ichthijosaiiri were spiral in form, as is the case with modern sharks. 

 He even found out what fish these ancient "sea dragons" had 

 been eating, inasmuch as scales of a fish called the Dajjedium 

 poUtwn were found in the coprohtes. In the same way, if my fox- 

 shark had become fossil, remains of mackerel might have been 

 found, giving evidence of what this creature had eaten. 



The Fox-shark is, I believe, more an Arctic animal 

 than the Beaumaris, porbeagle, or blue sharks, the kinds 

 generally found in the British seas. From the small size 

 of the teeth of the fox-shark, I conclude that it is more 

 formidable to small fish than to the larger inhabitants of 

 the sea. No one has yet been able to ascertain why the 

 tail is so remarkably prolonged. It is sometimes called 

 the "thresher shark," because it is said to inflict chas- 

 tisement on the whale though I can find no real autho- 

 rity for this being the case ; nor can I see why the 

 shark should flo^? the whale. The tail of the skate can 

 as I know to my cost, inflict severe wounds, and the 

 tail of the sting-ray contains a dagger which is even 

 more formidable still; I fancy, therefore, that the tail 

 of the fox- shark is used by the owner as a weapon of 

 defence, for if he chooses to use it, it would act like a 



