. The chordates. Chordata. ;m> Comparative Morphology of Chordates The dentition is exceptional. One pair of upper incisors become the tusks, more strongly developed in the male than in the female. The fully developed tusk consists of a somewhat modified dentine ("ivory") covered by a very thin layer of cement but lacking enamel except at the tip of a young tusk. A pair of temporary and relatively small "milk-tusks" precede the permanent tusks. In some extinct elephant-like mammals, a pair of lower incisors became a second pair of tusks, projecting downward. There are no
. The chordates. Chordata. ;m> Comparative Morphology of Chordates The dentition is exceptional. One pair of upper incisors become the tusks, more strongly developed in the male than in the female. The fully developed tusk consists of a somewhat modified dentine ("ivory") covered by a very thin layer of cement but lacking enamel except at the tip of a young tusk. A pair of temporary and relatively small "milk-tusks" precede the permanent tusks. In some extinct elephant-like mammals, a pair of lower incisors became a second pair of tusks, projecting downward. There are no canines. In the course of its lifetime, the animal acquires a total of 24 cheek teeth—on each side, 6 above and 6 below. Of the six produced in each half-jaw, never more than two, and at times only one, are in functional position at the same time (Fig. 575). Of the two present at a particular time, the anterior one is fully developed and its crown is completely exposed for use. The posterior one is incompletely developed and only the anterior part of its crown is exposed. Eventually the older anterior tooth is shed and then the next one, becoming fully developed, moves forward into its place. Meanwhile a partially developed tooth which had been buried in the jaw behind the functional two moves forward to become the posterior functional tooth. This replacement is repeated until the sixth. Fig. 575. Parasagittal section of elephant skull in a plane cutting through the root of one tusk, (ce) Cranial cavity; (Fr) frontal; (i) incisor tusk; (mi, m•:) first and second molar teeth; (Mx) maxillary; (n) nasal cavity; (Pa) parietal; (Prnx) pre- maxillary; (So) supraoccipital. The cranial cavity, containing the brain, occupies a small proportion of the volume of the skull. The size of the skull is greatly augmented by elaborate systems of air-filled sinuses, especially in its dorsal region and around the bases of the tusks. The arrangement of the bony walls of the sinuses is such as
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