The fifth molar, 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. The second root is more distinctly separated from the first simple root than from the large mass behind. It begins to appear above the gum about the twentieth year : its duration has not been ascertained by observation ; but it probably is not shed before the sixtieth year.
The sixth molar appears to be the last, and has from twenty-two to twenty-seven plates ; its length, or antero-posterior extent, following the curvature, is from twelve to fifteen inches : the breadth of the grinding surface rarely exceeds three inches and a half.
The reproductive power of the matrix in some cases surpasses that of the formative development of the cavity for lodging the tooth, and the last lamella: are obliged to be folded from behind forwards upon the side of the tooth. Fig. 99, p. 233. of my "History of British Fossil Mammals," shows this condition in the last lower molar of the Mammoth.
One may reasonably conjecture that the sixth molar of the Indian elephant, if it make its appearance about the fiftieth year, would, from its superior depth and length, continue to do the work of mastication until the pon derous Pachyderm had passed the century of its existence.
Mr. Corse has figured the sixth molar, (which he calls the seventh or eighth,) with twenty-three plates, in tab. x. of his Memoir, and a small cavity, c, is marked as an in cipient alveolus for a succeeding grinder. Had it actually been such, it might have been expected to contain some calcified portions of the anterior plates of such succeeding grinder.
The molar teeth, in all the species of elephant, succeed each other from behind forwards, moving, not in a right line, but in the arc of a circle, shown by the curved line in fig. 592. The position of the growing tooth in the closed alveolus (m. 5) is almost at right angles with that in use, the grinding surface being at first directed backwards in the upper jaw, and forwards 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 movement, the section dividing such whole into upper and lower portion runs parallel to the curve described by that move ment, the upper being the central portion, or that nearest the pivot, the lower, the pe ripheral portion : the grinding surface of the upper molars is consequently convex from behind forwards, 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 participates in this re volving course, advancing forwards with the teeth. The partition between the tooth in use and its successor is perforated near the middle ; and, in its progress forwards, that part next the grinding surface is first absorbed ; the rest disappearing with the absorption of the roots of the preceding grinder.
There are few examples of organs that manifest a more striking adaptation of a highly complex and beautiful structure to the exigencies of the animal endowed with it, than the grinding teeth of the elephant. We per ceive, for example, that the jaw is not en cumbered with the whole weight of the mas sive tooth at once, but that it is formed by degrees as it is required ; the division of the crown into a number of successive plates, and the subdivision of these into cylindrical pro cesses, presenting the conditions most favour able to progressive formation. But a more important advantage is gained by this sub division of the tooth ; each part is formed like a perfect simple tooth, having a body of dentine, a coat of enamel, and an outer in vestment of cement : a single digital pro cess may be compared to the simple canine of a Carnivore ; a transverse row of these, therefore, when the work of mastication has commenced, presents, by virtue of the different densities of their constituent sub stances, a series of cylindrical ridges of ena mel, with as many depressions of dentine, and deeper external valleys of cement : the more advanced and more abraded part of the crown is traversed by the transverse ridges of the enamel inclosing the depressed surface of the dentine, and separated by the deeper channels of the cement : the fore-part of the tooth exhibits its least efficient con dition for mastication ; the inequalities of the grinding surface being reduced in proportion as the enamel and cement which invested the dentinal plates have been worn away. This part of the tooth is, however, still fitted for the first coarse crushing of the branches of a tree: the transverse enamel ridges of the succeeding part of the tooth divide it into smaller fragments, and the posterior islands and tubercles of enamel pound it to the pulp fit for deglutition. The structure and pro gressive development of the tooth not only give to the elephant's grinder the advantage of the uneven surface which adapts the mill stone for its office, but, at the same time, secure the constant presence of the most efficient arrangement for the finer comminu tion of the food, at the part of the mouth which is nearest the fames.