TUSK OF NARWHAL. 



19 



ditioii in the young animal, those of the walrus are true 

 canines, whereas, as we have seen, the tusks of the 

 elephant are incisors. We have here, therefore, a well- 

 marked instance of the parallel development of severally 

 dissimilar structures to attain a marked general simi- 

 larity. The tusks of the walrus, which in old animals 

 attain a great length, are mainly employed in digging 

 up molluscs and crustaceans from the sand and shingle, 

 and also, it is said, to enable their owners to clamber up 

 on the ice. 



Although the whalebone-whales are entirely deficient in 

 teeth, and many of the dolphins and their allies have these 

 organs but poorly developed, there are two cetaceans 

 which exhibit a most remarkable development of tusks. 

 The first of these creatures is the well-known narwhal, 

 of the Arctic seas, in which, as a rule, there is one huge 

 spirally -twisted cylindrical tusk projecting from the left 

 side of the upper jaw of the male, which continues to grow 

 throughout life. Whether this solitary tusk is a canine 

 or an incisor, is not very easy to determine ; but it is 



FIG. 9. The Narwhal. (After True.) 



remarkable that its fellow of the opposite side generally 

 remains concealed in the jaw-bone, like the kernel of a nut 

 in its shell, while in the female both teeth are thus rudi- 

 mentary. Occasionally, however, male narwhals are met 

 with in which both the right and left tusks are developed ; 

 and it is somewhat curious that in such cases the direction 

 of the spiral in the two tusks is the same, instead of being, 

 as in the horns of antelopes, opposite. Although narwhals 

 have never been known to charge and pierce ships with 

 their tusks, after the manner of sword-fish, it is still 

 uncertain whether these formidable weapons (which may 

 attain a length of from eight to nine feet) are normally 



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