216 WHALES, PAST AND PRESENT xv 



(Pontoporia) there may be as many as sixty of such teeth on 

 each side of each jaw, making 240 in all. The more usual 

 number is from twenty to thirty. These teeth are never 

 changed, being " monophyodont," and they are, moreover, less 

 firmly implanted in the jaws than in land mammals, having 

 never more than one root, which is set in an alveolar socket, 

 generally wide and loosely fitting, though perfectly sufficient 

 for the simple purpose which the teeth have to serve. 



Most singular modifications of this condition of dentition 

 are met with in different genera of toothed whales, chiefly the 

 result of suppression sometimes of suppression of the greater 

 number, combined with excessive development of a single pair. 

 In one large group, the Ziphioids, although minute rudimentary 

 teeth are occasionally found in young individuals, and some- 

 times throughout life, in both jaws, in the adults the upper 

 teeth are usually entirely absent, and those of the lower jaw 

 reduced to two, which may be very large and projecting like 

 tusks from the mouth, as in Mesoplodon, or minute and entirely 

 concealed beneath the gums, as in Hyperoodon, an animal 

 which is for all practical purposes toothless, yet in which a 

 pair of perfectly formed though buried teeth remain through- 

 out life, wonderful examples of the persistence of rudimentary 

 and to all appearance absolutely useless organs. Among the 

 Delphinidce similar cases are met with. In the genus Grampus 

 the teeth are entirely absent in the upper, and few and early 

 deciduous in the lower jaw. But the narwhal exceeds all 

 other Cetaceans, perhaps all other vertebrated animals, in 

 the specialisation of its dentition. Besides some irregular 

 rudimentary teeth found in the young state, the entire denti- 

 tion is reduced to a single pair, which lie horizontally in the 

 upper jaw, and both of which in the female remain permanently 

 concealed within the bone, so that this sex is practically 

 toothless, while in the male the right tooth usually remains 

 similarly concealed and abortive, and the left is immensely 

 developed, attaining a length equal to more than half that of 

 the entire animal, projecting horizontally from the head in the 

 form of a cylindrical or slightly tapering pointed tusk, with 

 the surface marked by spiral grooves and ridges. 



The meaning and utility of some of these strange modifica- 



