TEETH OF UNGULATA. 363 



through the gum at the sixth year : it is in full use and place at 

 the fifteenth year (fig. 289, mi): the tooth is worn away, and its 

 last remnant shed, about the twentieth or twenty-fifth year. It is 

 the homologue of the first true molar of Hyrax, fig. 287, m i. 



The fifth molar, ib., m 2, with a crown of from seventeen to 

 twenty plates, measures between nine and ten inches in length, 

 and about three inches and a half in breadth. It begins to appear 

 above the gum about the twen- ^^^^ 292 



tieth year: its duration has not 

 been ascertained by observation. 



The sixth molar is the last, and 

 has from twenty-two to twenty- 

 seven plates ; its length, or an- 

 teroposterior extent, following 

 the curvature, is from twelve to 

 fifteen inches : the breadth of the **.m«n. 



grinding surface rarely exceeds three inches and a half. 1 



The molar teeth succeed each other from behind forward, 

 moving in the arc of a circle, shown by the curved line in fig. 

 289. The position of the growing tooth in the closed alveolus, 

 m 2, is almost at right angles with that in use, the grinding 

 surface being at first directed backward in the upper jaw, forward 

 in the lower jaw, and brought, by the revolving course, into a 

 horizontal line in both jaws, so that they oppose each other, 

 when developed for use. The imaginary pivot on which the 

 grinders revolve is next their root in the upper jaw, and is next 

 the grinding surface in the lower jaw ; in both, towards the 

 frontal surface of the skull. Viewing both upper and lower 

 molars as one complex whole, subject to the same revolving move- 

 ment, the section dividing such whole into upper and lower por- 

 tion runs parallel to the curve described by that movement, the 

 upper being the central portion, or that nearest the pivot, the 

 lower, the peripheral portion : the grinding surface of the upper 

 molars is consequently convex from behind forward, and that of 

 the lower molars concave : the upper molars are always broader 

 than the lower ones. 



The bony plate forming the sockets of the growing teeth is 

 more than usually distinct from the body of the maxillary, and 



1 In my ' Odontography ' I was led to conjecture that 4 this molar, if it makes its 

 appearance ahout the fiftieth year, would, from its superior depth and length, continue 

 to do the work of mastication until the ponderous Pachyderm had passed the cen- 

 tury of its existence : ' but I would now merely suggest, to all who may have the 

 opportunity, the desirability of making and recording observations supplementary to 

 those in the text made on captive Asiatic elephants in European menageries. 



