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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 to provide maximum 

 resistance to external impacts. The sinuses are in communication with the nasal cav- 

 ities. (Courtesy, Zittel: "Handbuch der Paleontologie," Munich, R. Oldenbourg. ) 



